Apparatus for removing silver from spent photoprocessing solution

ABSTRACT

Spent solutions from photographic processors are fed into a conduit (18) to which subsequently are added precipitating agents from a first source (22) and flocculating agents from a second source (28); so that, well grown or ripened clumps of flocculated solids are formed along the conduit before being emptied into a gravity collecting vessel and shipping container (38) from which clarified liquids are displaced by a mass (50) of accumulated flocculated solids, typically through a filter (58, 142, 150, 158, 162, 166, 220) for removal of any unsettled fines. Many of the clumps of flocculated solids become enlarged enough to individually extend across and substantially fill a cross-sectional area of the conduit. As a result, the enlarged clumps substantially block passage of and strain from the solution remaining fine particles which then adhere to the clumps.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 08/206,355 filedMar. 4, 1994 now U.S. Pat. No. 5,549,820.

TECHNICAL FIELD

The invention concerns apparatus and methods for removing unwanted orvaluable components from solutions. More particularly, the inventionrelates to removal of silver from spent solutions used to process ordevelop photographic film.

BACKGROUND ART

Processing or developing photographic film and paper products and otherimaging products requires the use of a variety of known types ofprocessing solutions. During use, the processing solutions graduallylose their effectiveness and must be replaced with fresh solutions.Photographic processors and film manufacturers for many years have beenconcerned with how to properly dispose of the spent or waste solutions.The spent solutions may contain precious metals such as silver whichoften have been recovered as an economic measure. Also, governmentregulations on discharge to the environment of solutions containing suchmetals typically have required that virtually all of the metals beremoved before the remaining liquid may be discharged to the sewer. Forexample, many state or local regulations in the United States restrictthe silver content of liquid discharged to a sewer, typically to a rangeof 1 to 12.5 parts per million (mg/L) of solution.

Certain ingredients in such spent solutions have been reacted withprecipitating and flocculating agents to form solid precipitatescontaining the metal or other ingredient to be removed. The precipitateshave been filtered from the solution and the remaining liquid has beendischarged. Some film processors have separated the precipitates bycentrifuging. Others have separated the precipitates by simple settlingand decanting. Various techniques of these sorts are described incommonly assigned U.S. Pat. No. 3,832,453 and by Thomas W. Bober andAustin C. Cooley in "The Filter Press for Filtration of InsolublePhotographic Processing Wastes," Photographic Science and Engineering,Vol. 16, No. 2, March-April 1972.

More recently, recovery of silver from spent solutions has been madesimpler due to introduction of metal salts, most commonly the trisodiumsalt of trimercapto-S-triazine or TMT, as the metal precipitating agent.Other cationic salts of TMT also can be used as precipitating agents,such as the potassium, ammonium or lithium salt. TMT can be used forprimary or secondary recovery of silver; however, many film processorshave reported that TMT is very useful and most economical for secondarytreatment of spent solutions which have previously had most of thesilver removed by conventional methods such as electrolysis, forexample. In one known method, the spent solution and TMT weremechanically mixed for as long as an hour in a large settling vessel,typically a round-bottomed or cone-bottomed vessel. The resultantmixture was left to settle overnight or for as long as twenty hours.Then, much of the liquid above the settled solids was decanted and thesettled solids were passed out of the bottom of the vessel into a bagfilter. Some work has been reported in the literature in which apolymeric flocculant has been added to the mixture prior to settling.Methods of the latter type were described by Nathan Spears and RobertSentell in a paper entitled "Silver Recovery from Photographic WasteProcessing Solutions by Using the Trisodium Salt of2,4,6-Trimereapto-S-Triazine," presented at the Seventh InternationalSymposium on Photo finishing Technology in San Francisco, Calif., 3 to 5Feb. 1992.

Those skilled in the photographic processing technologies willunderstand that various other types of components have been removed fromspent processing solutions by precipitation, such as Prussian blue (ironferrocyanide), calcium sulfate, various coupling agents, chromiumhydroxide from bleach and systems cleaners, aluminum salts and manyothers. Some of these precipitated materials tend to form rathergelatinous solids that will quickly clog or blind most filters. Othersproduce a very large mount of suspended fine particles that tend toremain suspended in the liquid even after rather long settling times.

While such known methods for removing components from wastephotoprocessing solutions have proven relatively effective at recoveryof precipitated solids, a number of problems have remained. Considerablecare has been required when decanting the last portions of the liquid inthe zone closest to the settled solids in the bottom of the settlingvessel, since the solids tend to stir up and carry out with the liquid,potentially requiring a further filtering operation or return of theliquid and fines from that zone to the vessel for processing with thenext batch of spent solutions.

Another problem may exist when solids already settled on the bottom ofthe settling vessel are disturbed when a valve is opened at the bottomof the vessel to dump the moist solids for further processing. If noliquid remains in the vessel when the valve is opened, difficulty may beencountered with getting the settled solids to flow completely out ofthe vessel without subsequent scraping or other manual handling. So,enough liquid often has been left in the vessel to permit the solids tobe discharged as a slurry for easy conveyance.

Similarly, there may occur times when it is desired to remove solidsfrom the settling vessel while a considerable volume of liquid remainsabove the settled solids. In such situations, when the solids aredisturbed by opening the bottom valve, some solids, particularly fines,are stirred up and resuspended in the liquid layer for a considerableperiod of time until the entire contents of the vessel are quiescent fora long enough period to allow the fines to settle again. But, if theliquid must be decanted before the fines have resettled, the fines arecarried out and a further filtering may be needed to achieve the desiredlevel of purity of the discharged liquid.

Various types of apparatus and methods are known for removingcontaminants from solutions such as waste water. However, the presentinventors' experience indicates that using such known apparatus andmethods for removal of silver from spent photoprocessing solutions wouldbe expected to produce effluents having silver concentrations farexceeding requirements of current government regulations. An example ofsuch known apparatus is found in U.S. Pat. No. 3,933,642 which disclosesa coiled tubular reactor for treating waste water solely byflocculation. The patent teaches a number of physical relationships saidto be required for a "decayed gradient flocculation" which is carriedout in very large scale waste water treatment installations. For flowrates and tubing diameters described in the patent, Reynolds numbers(N_(Re)) were calculated by the present inventors to be in excess of50,000 using the conventional formula Dvρ/μ, where D is pipe diameter, vis flow velocity, ρ is fluid specific gravity and μ is fluid viscosity.As will be shown later in the present specification, the flow regimesdescribed in the patent produce N_(Re) well beyond a desired maximum ofabout 4,000 experienced with embodiments of the present invention inwhich effluent silver concentrations were maintained below therequirements of government regulations.

To assess the efficacy of the system of U.S. Pat. No. 3,933,642 forsilver removal, bench-scale coiled reactors were assembled on behalf ofthe present inventors and tested using flow conditions (N_(Re),residence times, and ratio of tubing inside diameter to coil insidediameter) comparable with those of the patent, but employing theprecipitating and flocculating agents described later in the presentspecification. When these test reactors were operated using spentphotoprocessing solutions containing silver, effluent silverconcentrations exceeded those allowed by current regulations, eventhough for the tested reactors N_(Re) was lower by nearly an order ofmagnitude than N_(Re) calculated for the flow regimes specified by thepatent. Although some small masses of flocculated particles did form inthe test reactors, these masses would break up into even smaller masseswhen N_(Re) were approached which were only one tenth the magnitudecalculated for the flow regimes of the patent. As a result, many fineswere carried from the test reactor and the residual silver in theeffluent exceeded allowable standards. Thus, the present inventors wouldexpect that operating at the much higher N_(Re) of the flow regimespecified by the patent would produce still poorer performance and wouldnot solve the problems indicated previously.

SUMMARY OF THE INVENTION

The primary objective of the invention is to provide simple, compact andinexpensive apparatus and methods for removing a component from asolution, such as spent photo-processing solution, by treating thesolution to form settleable solids, passing the solids into a collectingvessel and allowing the remaining liquid to flow from the collectingvessel.

A further objective of the invention is to provide such apparatus andmethods in which the collecting vessel is readily removable and suitablefor use as a shipping container to transport the solids to anotherlocation for further processing, such as to a precious metals refiner.

Another objective of the invention is to provide such apparatus andmethods in which waste solutions from a film processing machine can betaken directly from the machine and treated in a reliable, repeatablemanner while maintaining clean surroundings, essentially withoutrequiting frequent intervention by the operator of the machine.

Still another objective of the invention is to provide such apparatusand methods in which removal of the collecting vessel causes minimaldisturbance to the operation of the rest of the apparatus or method.

Yet another objective of the invention is to accomplish separation ofsolids and liquids in a controlled manner in the shortest possible timefor greatest efficiency while minimizing any stirring up andresuspending of already settled solids and thus minimizing any need foradditional settling time.

A still further objective of the invention is to accomplish separationof solids and liquids in a reproducible manner and to such a degree thatthe clarified liquid will meet regulatory requirements for discharge orwill be suitable for reuse or reclamation.

Another objective of the invention is to accomplish such separationwhile decreasing the flow velocity of the solids and liquids as theymove through the apparatus thus enhancing the tendency of the solids toagglomerate into large clumps which will settle readily.

These objectives are given only by way of illustrative examples; thusother desirable objectives and advantages inherently achieved by thedisclosed invention may occur or become apparent to those skilled in theart. Nonetheless, the scope of the invention is to be limited only bythe appended claims.

The invention is defined by the appended claims. In a broad sense, anapparatus or method according to the invention is suited for removingsilver from spent photoprocessing solutions. Means or a step is providedfor combining a solution containing silver to be removed, aprecipitating agent for the silver, and a flocculating agent for aprecipitate formed by the solution and the precipitating agent. Aconduit is provided having a length, a cross sectional area, an inletend and an outlet end. Means or a step are provided for delivering thecombined solution at a flow rate into the conduit. The length of theconduit and the flow rate through the conduit are chosen to provide aresidence time sufficient for forming large, ripened clumps offlocculated particles of the precipitate. Uniquely in accordance withthe invention, the residence time further is chosen to be sufficient forallowing clumps of flocculated particles to grow and agglomerate duringpassage through the length into enlarged, apparently porous clumps manyof which individually extend across and substantially fill the crosssectional flow area, whereby the enlarged clumps substantially blockpassage of and strain from the solution remaining fine particles whichthen adhere to the enlarged clumps.

In one embodiment, the apparatus is particularly suited for continuouslyor intermittently removing silver from spent photoprocessing solution.Means are included for providing a solution containing silver to beremoved. Conduit means define a mixing path having a cross-sectionalarea, an inlet end and an outlet end, for receiving and passing thesolution. The mixing path may be a closed conduit such as a length oftubing through which the solution is pumped. First means, such asgravity feed or a peristaltic or bellows pump, is provided fordelivering the solution into the inlet end of the mixing path. Secondmeans, such as gravity feed or a suitable pump, is provided downstreamof the first means for delivering into the conduit means a precipitatingagent for the component. The spent solution and precipitating agent alsomay be delivered to the conduit means in the reverse order oressentially simultaneously, provided the proper ratios are maintained.In some embodiments of the invention, the spent solution andprecipitating agent may be mixed in a separate vessel and the mixturedelivered into the conduit means. Third means, again such as gravityfeed or a suitable pump, is provided downstream of the second means fordelivering into the conduit meaning a flocculating agent for theprecipitate. The second and third means are separated by a firstdistance chosen to provide a first residence time sufficient for mixingof the solution and the precipitating agent and for forming aprecipitate well suited for flocculation. That is, the residence timebetween the second and third means is long enough to enable the crystalsof precipitate to grow or ripen to a point at which addition of aflocculating agent will cause formation of flocculated particles whichtend to agglomerate readily into clumps. In some applications, however,the residence time for precipitation may be very short. The outlet endof the conduit means is located downstream of the third means, at asecond distance chosen to provide a second residence time sufficient forforming larger, more ripened clumps of flocculated particles of theprecipitate. The second residence time also is sufficient to allow themore ripened clumps to grow and agglomerate during passage through thesecond distance into enlarged, apparently porous clumps, many of whichindividually extend across and substantially fill the cross-sectionalflow area. As a result, the enlarged clumps substantially block passageof and strain from the solution remaining fine particles which thenadhere to the large clumps.

A collecting and shipping vessel having an inlet, releasably connectedto the outlet end of the conduit means, may be provided for receivingthe flocculated solids and any remaining liquid and for permitting theflocculated solids to settle to a bottom of the vessel and the remainingliquid to move toward an outlet of the vessel. As a result, the settledflocculated solids gradually will substantially fill the vessel while atleast a substantial part of the remaining liquid gradually will passfrom the vessel, thereby permitting a filled collecting and shippingvessel to be disconnected from the outlet end of the conduit means. Theflow area of the collecting and shipping vessel preferably issubstantially larger than that of the conduit means, thereby causing thesolution velocity to decrease and the solids to settle more readily inthe vessel. The collecting and shipping vessel may be used as a shippingcontainer for the settled solids.

One embodiment of the method of the invention is suited for continuouslyor intermittently removing silver from spent photoprocessing solution. Asolution containing silver to be removed is provided, either from aholding vessel or directly from a photo-processing machine. A mixingpath such as an elongated tube is defined having a cross-sectional area,an inlet end and an outlet end for receiving and passing the solution;and the solution is delivered into the inlet end. Downstream of thepoint of delivery of the solution, a precipitating agent for thecomponent is delivered into the mixing path. The spent solution andprecipitating agent also may be mixed in a separate vessel beforedelivery into the elongated tube. Downstream of the point of delivery ofthe precipitating agent at a first distance chosen to provide a firstresidence time sufficient for mixing of the solution and theprecipitating agent and for forming a precipitate well suited toflocculation, a flocculating agent for the precipitate is delivered intothe mixing path. The outlet end of the mixing path preferably isdownstream of the point of delivery of the flocculating agent at asecond distance chosen to provide a second residence time sufficient forforming larger, more ripened dumps of flocculated particles ofprecipitate. The second residence time also is sufficient to allow themore ripened clumps to grow and agglomerate during passage through thesecond distance into enlarged, apparently porous clumps, many of whichindividually extend across and substantially fill the cross-sectionalflow area. As a result, the enlarged clumps substantially block passageof and strain from the solution remaining fine particles which thenadhere to the large clumps.

The flocculated solids and any remaining liquid are collected in a firstcollecting and shipping vessel having an inlet, releasably connected tothe outlet end of the mixing path, for receiving the flocculated solidsand any remaining liquid. The flocculated solids are permitted to settleto a bottom of the first vessel and the remaining liquid to move towardan outlet of the first vessel. The settled flocculated solids graduallywill substantially fill the first vessel while at least a substantialpart of the remaining liquid gradually will pass through the outlet fromthe first vessel, thereby permitting a filled first vessel to bedisconnected from the outlet end of the mixing path. When the firstvessel has filled, it is removed from communication with the outlet end;and a second, empty vessel is connected to the outlet of the mixingpath.

In the previously described apparatus and method of the invention, theconduit means may increase in flow area from the inlet end to the outletend to thereby decrease the flow velocity of the flocculated solids andenhance their tendency to agglomerate and separate from the liquid. Anintermediate settling vessel may be provided for receiving theflocculated solids and any remaining liquid from the conduit means, thesettling vessel having a sloped bottom wall and a bottom outlet forliquid and flocculated solids, the collecting vessel being releasablyconnected to the bottom outlet of the settling vessel. The settlingvessel has a cross section and may comprise an internal baffle wallextended across a chord of the cross section, the baffle wall having alower edge near the sloped bottom wall and the conduit means extendinginto the settling vessel on one side of the baffle wall; and an outletfor clarified liquid on an opposite side of the baffle wall near anupper end of the settling vessel. In some applications where fineparticles of precipitate do not settle from the settling vessel into thecollecting vessel, a further collecting vessel may be connected to thisoutlet for clarified liquid, to remove such fines. The conduit means mayinclude static mixing elements between the second and third means fordelivering. Optionally, static mixing elements may be includeddownstream of the third means for delivering. The conduit means may becurved into a fiat coil, a helix, a spiral, a flattened helix or spiral,undulating pattern or other regular or irregular patterns.

A further embodiment of the apparatus of the invention includes meansfor providing a spent photoprocessing solution containing silver to beremoved; a mixing vessel; and first means for delivering the solutioninto the mixing vessel. Second means are provided for delivering aprecipitating agent for the silver into the mixing vessel. Third meansare provided for delivering a flocculating agent for the precipitateinto the mixing vessel, whereby flocculated solids are formed by theprecipitate and the flocculating agent. A settling vessel is includedfor receiving flocculated solids and any remaining liquid from themixing vessel, the settling vessel preferably having a sloped bottomwall and a bottom outlet for liquid and flocculated solids. An internalbaffle wall is extended across a chord of the settling vessel, with thebaffle wall extending from an upper end of the settling vessel downwardto near the sloped bottom wall, thereby defining an inlet passage on oneside of the baffle wall within the settling vessel, the inlet passagehaving a length sufficient to provide adequate residence time forformation and some ripening of clumps of the flocculated solids. Theinlet passage also may be defined by a downwardly extending conduit ornest of conduits within the settling vessel, rather than by a bafflewall. An outlet for clarified liquid is provided on an opposite side ofthe baffle wall near an upper end of the settling vessel. Fourth meansare provided for delivering flocculated solids and liquid from themixing vessel into the inlet passage.

A collecting and shipping vessel having an inlet, releasably connectedto the bottom outlet of the sealing vessel, preferably is provided forcontinuously or intermittently receiving the flocculated solids and anyremaining liquid, for permitting the flocculated solids to settle to abottom of the collecting and shipping vessel and the remaining liquid tomove toward an outlet of the collecting and shipping vessel, whereby thesettled flocculated solids gradually will fill substantially thecollecting and shipping vessel while at least a substantial part of theremaining liquid gradually will pass from the vessel, thereby permittinga filled collecting and shipping vessel to be disconnected from thebottom outlet of the settling vessel.

A further embodiment of the method of the invention may include thesteps of providing a spent photoprocessing solution containing silver tobe removed; providing a mixing vessel; delivering the solution into themixing vessel; delivering a precipitating agent for the silver into themixing vessel; delivering a flocculating agent for the precipitate intothe mixing vessel, whereby flocculated solids are formed by theprecipitate and the flocculating agent; providing a settling vessel forreceiving flocculated solids and any remaining liquid from the mixingvessel, the settling vessel preferably having a sloped bottom wall and abottom outlet for liquid and flocculated solids; providing an internalbaffle wall extended across a chord of the settling vessel from an upperend of the settling vessel downward to near the sloped bottom wall,thereby defining an inlet passage on one side of the baffle wall withinthe settling vessel, the inlet passage having a length sufficient toprovide adequate residence time for formation and some ripening ofclumps of the flocculated solids.

The method further comprises providing an outlet for clarified liquid onan opposite side of the baffle near an upper end of the settling vessel;delivering flocculated solids and liquid from the mixing vessel into theinlet passage; delivering flocculated solids and liquid from thesettling vessel into a first shipping and collecting vessel having aninlet, releasably connected the bottom outlet of the settling vessel,for receiving the flocculated solids and any remaining liquid;permitting the flocculated solids to settle to a bottom of thecollecting and shipping vessel and the remaining liquid to move towardan outlet of the collecting and shipping vessel, whereby the settledflocculated solids gradually will fill substantially the collecting andshipping vessel while at least a substantial part of the remainingliquid gradually will pass from the collecting and shipping vessel,thereby permitting a filled collecting and shipping vessel to bedisconnected from the bottom outlet of the settling vessel; removing thefirst collecting and shipping vessel when it is filled; and connecting asecond, empty collecting and shipping vessel to the outlet of the mixingpath.

In any of the previously described apparatus and methods of theinvention, the outlet of the collecting and shipping vessel may be abovethe bottom of the vessel; and the flocculated solids and any remainingliquid may flow into the vessel near the bottom of the vessel, wherebyany remaining liquid flows upward through previously settled solids,thereby removing fines before the liquid reaches the outlet of thevessel. By "near the bottom of the vessel" is meant that flocculatedsolids and liquid are flowed into the collecting and shipping vesselclose enough to the bottom to avoid undue breaking up of the clumpsentering or already resting in the vessel or excessive stirring up offines. The clearance to the bottom of the vessel may be adjusteddepending on the spent solution being treated. The collecting andshipping vessel may comprise a filter for removing fines from liquidflowing through the outlet of the collecting and shipping vessel. Thefilter may be a porous bag suspended within the collecting vessel, theflocculated solids being captured within tie bag. When a filter bag isused, the primary mode of separation is setting within the bag; thesecondary mode of separation is filtration through the bag; and thetertiary mode is wicking of liquid by the bag. When the outlet from thecollecting vessel is located above the bottom of the vessel, the filtermay comprise an annular ring of filter material supported at the levelof the outlet; and the inlet of the collecting vessel may open insidethe annular ring, whereby flocculated solids settle to the bottom of thecollecting vessel and remaining liquid eventually rises to flow throughthe annular ring to the outlet.

Also in any of the previously described apparatus and method of theinvention, the flocculated solids and any remaining liquid flow into thecollecting and shipping vessel near the bottom of the vessel, wherebyany remaining liquid must flow upward through previously settled solids,thereby removing fines from the liquid; a filter element is positionedwithin the collecting vessel, the filter element dividing the interiorof collecting vessel into a first chamber for receiving flocculatedsolids and liquid and a second chamber for receiving liquid passedthrough the filter element; and the outlet of the collecting andshipping vessel is connected to the second chamber. Preferably, thefirst chamber is substantially larger in volume than the second chamber.The filter element may extend upward from the bottom of the collectingand shipping vessel and may be tubular; and the second chamber may besurrounded by the filter element. The filter element may be tubular andthe first chamber may be defined within the filter element. The filterelement may be tubular and the second chamber may be defined within thefilter element.

In accordance with a further aspect of the invention, an apparatus forcollecting and separating flocculated solids and liquid may include acollecting and shipping vessel having an interior and a bottom; an inletfor flocculated solids and liquid to flow into the vessel near thebottom, whereby the liquid must flow upward through previously settledsolids, thereby helping to remove fines from the liquid; a filterelement positioned within the vessel, the filter element dividing theinterior into a first chamber for receiving flocculated solids andliquid from the inlet and a second chamber for receiving liquid passedthrough the filter element; and an outlet for the liquid to flow fromthe second chamber.

In accordance with yet a further aspect of the invention, a method forcollecting and separating flocculated solids and liquid may include thesteps of providing a collecting and shipping vessel having an interiorand a bottom; flowing flocculated solids and liquid into the collectingvessel near the bottom, whereby the liquid must flow upward throughpreviously settled solids, thereby removing fines from the liquid;positioning a filter element within the collecting vessel, the filterelement dividing the interior into a first chamber for receiving theflow of flocculated solids and liquid and a second chamber for receivingliquid passed through the filter element; and flowing the filteredliquid from the second chamber.

In the further aspects of both the apparatus and method of theinvention, the filter element may extend upward from the bottom and maybe tubular; and the second chamber may be surrounded by the filterelement. The filter element may be tubular and the first chamber may bedefined within the filter element. The filter element may be tubular andthe second chamber may be defined within the filter element.

The apparatus and methods of our invention provide numerous significantadvantages over the prior art. The apparatus is very versatile and workswith a great variety of solutions to be treated and ingredients to beremoved or recovered. It is dean, not messy, and eliminates directhandling of chemical precipitates, chemically coated filters andreaction chemicals. Automatic metering is used so that manual measuringof individual reactants for various batch sizes is not required. Theapparatus is inexpensive to make and has a compact size, requiringminimal floor space ("footprint") compared to prior art equipment. Nolarge solution storage tanks are required since a continuous orintermittent method is used rather than batch. No large settling orprecipitation tanks are needed, which in conventional systems mayrequire from several hours to several weeks to accomplish satisfactorysettling, depending on composition. The reaction conduit and collectionvessels do not need to be open to atmosphere. Therefore, with ourinvention there is a minimum of odors and contamination of solutions,worker exposure to vapors, corrosion of nearby equipment and facilitiesor need for large ventilating systems. Any objectionable gases such asammonia which might be encountered are typically retained within thesystem. The collection vessel is inexpensive and also acts as theshipping vessel for precipitated solids. The method greatly extends thelife of the filter used since settling is the primary mode of solidsseparation and filtration is only a secondary mode, even though done inthe same vessel. Therefore a much greater quantity of solution may bepassed through this system compared to conventional filtration. Theapparatus is simple, having very few moving parts and easily replaceablecomponents and therefore requires minimal maintenance and downtime. Themethod and apparatus provide real-time treatment of solution bysimultaneous treatment steps at various zones in the system; therefore,each increment of solution receives essentially the same treatmentregardless of when it enters the system. The method is highlyreproducible for a given composition of solution. Use of the method maysimplify compliance with certain hazardous chemical storage regulationsby minimizing the of amount and duration of storage of such hazardousmaterials.

Other advantages are provided by our invention. The apparatus is easy touse by operators unskilled in chemical technology. The apparatus easilymay be made portable, easily transportable between sites by one person,and not dependent on fixed supporting utilities except for an electricsource. A portable embodiment can be taken to the photographic or otherprocess as needed and returned to storage when not needed. The apparatusprovides totally automatic operation, is able to operate unattended atall hours including overnight, and starts and stops automatically asnecessary. The apparatus and method provide low operating pressures.Extremely safe operation is provided due to relative lack of chemicalspills and splashes since all reactants and products are contained. Onlysmall quantities are being processed at a given time which precludesdisastrous spills. The invention provides an inexpensive method,including materials and labor, of separating and transporting solidscompared to existing techniques. The apparatus minimizes the potentialto generate fine solid particles which normally tend to confoundconventional settling and filtration methods. The invention features abuilt-in dewatering step which promotes compaction of the flocculatedsolids and concentration of the recovered silver to eliminate unwantedwater, which thereby decreases shipping and recovery or treatment costs.The collecting/shipping vessel may be totally combustible, makingrefining of recovered metals less complicated and therefore less costly.The apparatus permits better estimates of the recovered silver in theshipping container by the equipment user, due to transparency ortranslucency of the vessel, consistency of collected product, andimproved ability to estimate value of contents from weight, therebyensuring that user will get a fair price from the refiner. The apparatusand method effectively operate over all typical effluent ranges ofsilver typically encountered in photographic processing wastes, frommilligrams per liter to tens of grams per liter, using a singleapparatus. This is compared to other conventional recovery systems whichmay require two or more systems in tandem, one for primary recovery torecover economically higher levels and the second for secondary recoveryto reduce residuals to environmentally acceptable low levels. Theinvention permits flocculated solids, once collected, to remainundisturbed after collection, thereby eliminating separate secondaryrecovery operations. The apparatus can be operated by gravity flow ifdesired thereby saving costs of pumps and electricity. Therefore, theapparatus could readily be adapted for use in remote locations that donot have electricity.

Still other advantages are provided by our invention. The readyreleasability and changeover of collecting vessels minimizes downtimeand complexity of operation. The invention permits easy handling ofreagents and collection vessels because of the relatively small sizesinvolved and the sealed nature of the collection vessels. The apparatustypically produces higher density flocculated solids in a given timethan in conventional clarifiers because of the solids agglomeration orcompaction mechanism inherent to the method. The reaction conduit iseasily visually observable in operation to immediately discern andcorrect any operational problems. Since many applications of theinvention will be in a retail environment, the apparatus can be enclosedeasily in a simple and clean-appearing housing, which does not suggestan industrial treatment process. The simplicity and small number ofcomponents of the apparatus permit arrangement of the components in awide variety of external geometries for different user circumstances.Batches of photographic processing or other solutions containing avariety of concentrations of silver or other ingredients are homogenizedto a considerable degree and their concentrations damped out to moreuniform concentrations, for more uniform treatment and greater reagentcost savings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objectives, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments of the invention, asillustrated in the accompanying drawings.

FIG. 1 illustrates schematically an apparatus for removing silver fromspent photo-processing solutions in accordance with the invention, inwhich the unique enclosed reaction conduit in accordance with theinvention is formed into a helical coil.

FIG. 2 illustrates schematically an embodiment of the reaction conduitwhich is formed into a horizontally undulating pattern.

FIG. 3 illustrates schematically an embodiment of the reaction conduitwhich is formed into a vertically undulating pattern.

FIG. 4 illustrates schematically an embodiment of the reaction conduitwhich is formed into a spiral coil.

FIG. 5 illustrates schematically a perspective view of an embodiment ofthe reaction conduit which is formed into a flat coil.

FIG. 6 illustrates schematically a plan view of the reaction conduit ofFIG. 5.

FIG. 7 illustrates schematically an apparatus for removing silver fromspent photo-processing solutions in accordance with the invention, inwhich the reaction conduit comprises successive sections of tubing whichincrease in flow area between the inlet and outlet of the reactionconduit.

FIG. 8 illustrates schematically an embodiment of the reaction conduitwhich is curved into a vertically undulating pattern of conduits whoseflow areas increase between the inlet and outlet of the conduit.

FIG. 9 illustrates schematically a settling vessel and collecting vesseluseful in accordance with the invention.

FIG. 10 illustrates schematically the collecting vessel of FIG. 9 withan overflow line and container for expelled liquid.

FIG. 11 illustrates schematically an application of the reaction conduitof FIGS. 5 and 6 with a settling vessel of FIG. 9.

FIG. 12 illustrates schematically an apparatus for removing silver fromspent photo-processing solutions in accordance with the invention, inwhich the reaction conduit is a tube with internal static mixingelements, the tube being extended into a settling vessel of FIG. 9.

FIG. 13 illustrates schematically an apparatus for removing silver fromspent photo-processing solutions in accordance with the invention, inwhich the spent solutions, precipitating agent and flocculating agentare mixed in a vessel and the mixture is pumped through a conduit into asettling vessel of FIG. 9.

FIGS. 14 to 20, 30 and 31 illustrate schematically various embodimentsof the collecting vessel according to the invention.

FIGS. 21 to 29 illustrate schematically various embodiments foradmitting flocculated solids and liquid to a collecting vessel and forwithdrawing separated liquid.

FIG. 32 illustrates schematically an apparatus for removing silver fromspent photo-processing solutions in accordance with the invention, inwhich the collecting vessel of FIG. 30 is used

FIG. 33 illustrates schematically an apparatus for removing silver fromspent photo-processing solution in accordance with the invention, inwhich the spent solution and precipitating agent are combined in amixing vessel and the mixture is pumped into a reaction conduit whereflocculating agent is added.

FIG. 34 illustrates schematically a large-scale apparatus in accordancewith the invention, in which the reaction conduit is divided into aninitial coiled section having an essentially horizontal axis and,beneath the initial section, a final coiled section having anessentially vertical axis.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following is a detailed description of the preferred embodiments ofthe invention, reference being made to the drawings in which the samereference numerals identify the same elements of structure in each ofthe several Figures.

FIG. 1 illustrates an apparatus 10 which functions in accordance withone embodiment of the method of the invention. The apparatus is usefulfor removing a variety of components from solutions, but is particularlyuseful for primary or secondary removal of silver from spentphoto-processing solutions. An infeed conduit 12 is provided to deliverspent solutions from a holding tank, not illustrated, or directly from aphoto-processing machine. The solutions are drawn through conduit 12 bya first pump 14 for delivering the solutions to the inlet end 16 of areaction conduit means 18 which defines a mixing path. In someapplications of the invention, pump 14 may be eliminated and the spentsolution may be delivered from a tank, not illustrated, positioned toprovide an adequate gravity head for flow into conduit 12. Conduit means18 may be conventional flexible tubing such as transparent plastictubing or the like and may be formed as illustrated into a helical cotto enhance mixing and to provide a compact arrangement. The axis of thecoil may be generally vertical, as illustrated, or horizontal, or partlyvertical and partly horizontal, or at any intermediate orientation. Thecoil may be open or flattened and its perimeter may have any regular orirregular shape. A check valve 20 optionally may be provided at theoutlet of pump 14. A source 22 of a suitable precipitating agent, suchas a solution of TMT for removal of silver, is connected to a secondpump 24 for delivering the precipitating agent into conduit means 18 ata point just downstream of inlet end 16 and check valve 20. The spentsolution and precipitating agent also may be delivered to the conduitmeans in the reverse order or essentially simultaneously, provided theproper ratios are maintained. A check valve 26 optionally may beprovided downstream of pump 24 at an inlet 27 to the conduit means. Asource 28 of flocculating agent is connected to a third pump 30 fordelivering the flocculating agent into conduit means 18. A suitableflocculating agent for removal of silver is a cationic copolymer ofacrylamide and acryloyloxyethyl trimethyl ammonium chloride availablefrom the Calgon Corporation as Product No. POL-E-Z-2406. Commonlyassigned U.S. Pat. No. 5,437,792 discloses various compositions of sucha flocculating agent. A check valve 32 optionally may be provideddownstream of pump 30 at an inlet 33 to the conduit means.

The flow from pump 30 is delivered into conduit means 18 downstream frominlet 27 from pump 24 at a distance chosen to provide a residence timesufficient for mixing the spent solutions and the precipitating agentand for forming a precipitate well suited for flocculation. That is, theresidence time is long enough to enable crystals of precipitate to growor ripen to a point at which addition of a flocculating agent will causeformation of flocculated particles which tend to agglomerate intoclumps. Different spent solutions may require different residence times.The length of conduit can be readily determined experimentally forvarious spent solutions by those skilled in the art. Once the residencetime has been determined for a given spent solution, reproducibleresults can be achieved in accordance with the invention. Conventionalstatic mixing elements, such as those disclosed in U.S. Pat. No.3,286,992, may be installed in conduit means 18 upstream or downstream,or both, of inlet 33 to facilitate good mixing. However, in manyapplications ordinary flow of spent solutions and precipitating agentwill provide adequate mixing without static mixing elements. In theillustrated embodiment, pumps 14, 24 and 30 may be conventionalperistaltic, diaphragm or bellows pumps or the like and may be driven bya common motor 34 to synchronize the pulses of liquid into conduit 18,thus potentially eliminating any need for check valves 20, 26, 32.Alternatively, if them is sufficient difference in elevation amongreaction conduit means 18 and sources 22, 28 to provide adequate gravityhead for flow into reaction conduit means 18 to produce suitable mixingof spent solution, precipitating agent and flocculating agent, thenpumps 24, 30 may be eliminated without departing from the scope of theinvention. Also, the downward direction of flow through conduit means 18illustrated in FIG. 1 may be reversed to upward flow, as will bediscussed further with regard to FIG. 4.

The outlet end 36 of conduit means 18 preferably is downstream frominlet 33 from pump 30 at a distance chosen with regard to the flow ratethrough conduit means 18 to provide a residence time sufficient forforming large, ripened clumps of flocculated particles of theprecipitate. The residence time also is sufficient to allow the moreripened clumps to grow and agglomerate during passage through theconduit means into enlarged, apparently porous clumps. In this and otherembodiments of the invention, many of these enlarged, apparently porousclumps individually extend across and substantially fill thecross-sectional flow area. These enlarged clumps move through theconduit means, sometimes rolling or tumbling. As a result, many of theseenlarged clumps tend to scrub from the walls of the conduit meansprecipitated fines which may have adhered there. These enlarged clumpswhich fill the flow area also tend to substantially block passage of andstrain from the liquid remaining fine particles, which then adhere tothe enlarged clumps. Due to the agglomerating and straining, the densityof the clumps appears to increase as they move through the conduit.Those skilled in the art will appreciate that various combinations andconcentrations of spent solutions, precipitating agent and flocculatingagent will produce flocculated solids having different characteristicsand requiring different residence times to form the desired enlargedclumps which will control passage of fines and settle properly. As inthe case of residence time for mixing of spent solution andprecipitating agent, the length of conduit required for formation ofsuch clumps can be readily determined experimentally for variousanticipated combinations, after which reproducible results can beachieved.

The enlarged clumps of flocculated particles and remaining liquid flowupward from the lower end of conduit 18 and through outlet end 36 into apreferably enclosed collecting and shipping vessel 38 in accordance withthe invention. Upward movement to outlet 36 helps with formation of thepreviously described enlarged clumps which substantially fill thecross-sectional flow area. In this and the other embodiments of theinvention, reaction conduit 18 preferably should be kept full ofsolution at all times. Collecting vessel 38 preferably is small enoughto be handled manually, sealed and shipped when full of flocculatedsolids and some liquid. A one to ten gallon (3.785 to 37.85 L) containerwill suffice for many applications, but smaller or much largercontainers also may be used. Collecting vessel 38 typically will becylindrical but may have many shapes including a bottom 40, side wall(s)42 and a preferably sealed top 44. A removable, threaded cap or closure46 is provided to manually, releasably connect conduit means 18 tovessel 38. Closure 46 has a through passage to which conduit means 18 isconnected in any convenient manner, such as those to be discussedsubsequently. Within vessel 38, a conduit 48 preferably extends belowclosure 46 to deliver the flocculated solids and liquid to a locationnear bottom 40. Thus, as the flocculated solids and liquid flow into thevessel, a mass 50 of solids collects on the bottom and a layer or volume52 of separated liquid collects above mass 50. By "near the bottom ofthe vessel" is meant that flocculated solids and liquid are flowed intothe collecting vessel close enough to the bottom to avoid undue breakingup of the clumps entering or already resting in the vessel or excessivestirring up of fines. The clearance to the bottom of the vessel may beadjusted depending on the spent solution being treated. When asufficient mass 50 has accumulated on bottom 40 to rise above the lowerend of conduit 48, the subsequently discharged flocculated solids andliquid will be forced to flow into mass 50, where the flocculated solidsand much of the fines will be caught due to the self-filtering effect ofmass 50. Liquid will rise through mass 50 without much disturbing thesettled flocculated solids. The level of the clarified liquid rises toincreasingly higher levels until the liquid leaves the vessel throughanother passage in closure 46, which may be connected to an optionalcheck valve 54 provided just downstream of closure 46 in a dischargeconduit 56. A final filter 58 may be provided in conduit 56 if necessaryto remove additional fines before the liquid is discharged from theapparatus, such as to the sewer. FIGS. 14 to 31 illustrate alternateembodiments of collecting vessel 38 and modes for its attachment toreceive flocculated solids and liquid. As can be understood from FIGS. 1and 14 to 31 and their associated descriptive passages in thisspecification, collecting and shipping vessel 38 is closed, other thanat the means for manually, releasably connecting the conduit forincoming liquid and solids and a port for a conduit for outgoing liquid.As illustrated, while the collecting and shipping vessel is connected tothe overall apparatus, the vessel's being closed in this mannereffectively prevents removal of settled flocculated solids fromlocations within the vessel which are below the level of the liquid inthe vessel. As a result of this arrangement, the settled flocculatedsolids progressively accumulate, agglomerate and compact, trappingresidual fines, expelling entrapped liquid and forcing the liquid upwardfrom the agglomerated mass 50 toward the outlet of the vessel.

Use of collecting vessel 38 is preferred in accordance with theinvention to provide optimum assurance that flocculated solids, onceformed and ripened within conduit means 18, will be able to settle to alocation where they subsequently will be disturbed very little and willbe able to agglomerate into mass 50. As the higher specific gravity mass50 forms and agglomerates on bottom 40, the lower specific gravityliquid which entered with the flocculated solids is gradually displacedupward in the vessel until it reaches discharge conduit 56. Then, whenvessel 38 is full, incoming flow is stopped in any of the manners to bediscussed shortly. The vessel is removed, sealed and replaced by anidentical empty vessel. The full vessel may then be shipped away forfurther processing, such as to a refiner for recovery of precious metal.

EXAMPLE 1

A fixer/bleach-fixer/stabilizer/low-flow wash mixture from the KODAKFlexicolor film process C-41 and KODAK Ektacolor paper process RA-4,containing 3 grams/liter silver, was used as the feed solution in theapparatus shown in FIG. 1. Reaction conduit means 18 was a horizontal,flattened helix configuration having a generally oval shape. A 15% byweight solution of TMT (TMT-15) at a concentration of 305 gm/L in waterwas the precipitating agent in source 22. American Cyanamid Magnifloc846A at a concentration of 1.0 gm/L in water was used as theflocculating agent in source 28. The total flow of solutions was asfollows: 308 ml/min of spent solutions from pump 14, 6.15 ml/min ofprecipitating agent from pump 24, and 6.15 ml/min of flocculating agentfrom pump 30. Conduit 18 was a flattened helical coil of flexibletransparent polyvinyl chloride tubing having a 0.375 inch (9.53 mm)inside diameter. Pumps 14, 24 and 30 were peristaltic pumps operatedsimultaneously on a common shaft by a 100-rpm motor 34. Collectingvessel 38 was a commercial 5-gallon (18.93 L) sealed polyethylene canfor liquid shipment with top openings, of a translucent nature whichallowed the operation and contents to be observed as the can filled withsolids and liquid. Optional check valves 20, 26 and 32 were not used.Spent solution was injected into inlet end 16 at the side leg of a nylontee, with precipitating agent injected into inlet 27 at an end leg ofthe same tee. The tee exited into conduit 18 containing a static mixersection immediately downstream of inlet 27. Initially, the static mixersection was tested at 24 inches (610 mm) in length, but experimentationfor this particular combination of spent solutions and precipitatingagent established an optimum length in this trial to be 8 inches (203mm). A residence time downstream of the static mixing section sufficientfor forming the silver-TMT precipitate initially was provided by afurther conduit length of 26 inches (660 mm) downstream of the staticmixer section; but further experimentation established this length to bemore optimum at 20 inches (508 mm). Flocculating agent from source 28was then injected through a second nylon tee into conduit 18 at inlet 33from pump 30, the point of injection being approximately 28 inches (711mm) downstream of inlet end 16. A second static mixer section, firsttested at 8 inches (203 mm) long and later optimized at 4.5 inches (114mm) long was placed in conduit 18 immediately downstream of inlet 33. Aresidence time downstream of the second static mixer section sufficientfor forming clumps of particles of precipitate was provided by a furtherconduit length of about 58 inches (1473 mm) which provided approximately2 minutes residence time for growth or ripening of clumps.

The flocculated solids formed in conduit 18 became enlarged, pea-sizedclumps of yellow material with a mucoid consistency. The residence timewas sufficient to allow the more ripened clumps to grow and agglomerateduring passage through the conduit into enlarged, apparently porousclumps. Many enlarged clumps individually extended across andsubstantially filled the cross-sectional flow area. The enlarged clumpsmoved through the conduit, sometimes rolling or tumbling. As a result,many of the enlarged clumps tended to scrub from the walls of theconduit precipitated fines which had adhered there. The enlarged clumpswhich filled the flow area also tended to substantially block passage ofand strain from the liquid remaining free particles, which then adheredto the enlarged clumps. Due to the agglomerating and straining, thedensity of the clumps appeared to increase as they moved through theconduit. These clumps were discharged into collecting vessel 38 througha 0.250 inch (6.35 mm) conduit 48 which ended about 0.5 inch (12.7 mm)above bottom 40. The mass of solids 50 which gradually built up aroundthe end of conduit 48 served as a filtering medium to remove residualfines. Liquid effluent from vessel 38 passed through a bag filter 58 asa final polishing filter which removed any remaining fines in theliquid. The apparatus was permitted to operate intermittently forseveral days, to simulate actual operating conditions in a photographicprocessing laboratory. As the mass of material sat undisturbed in thecollecting vessel for several weeks, small pockets of liquid that hadstill been entrapped in the settled solids were expelled upward, untilthe bottom solids were a relatively homogeneous solid yellow mass. Thefinal silver concentration in the clarified liquid was less than 1 mg/L.

FIG. 2 illustrates an alternate embodiment of conduit means 18. Ratherthan the simple helical coil of conduit as in FIG. 1 or the flattenedhelix of Example 1, conduit means 18 can be arranged in an undulating,back and forth, rather sinusoidal pattern in which straight spans 60 ofconduit are essentially horizontal between the connecting turns orcurved portions 62. The changes of direction of the flow help to promoteproper mixing. If desired for faster or more thorough mixing,conventional static mixing elements 64 of the type previously mentioned,may be installed in conduit means 18 between the inlet for precipitatingagent and the inlet for flocculating agent, as indicated schematicallyby the cross-hatched areas. In some cases, static mixing elements mayalso be used after inlet 33 for flocculating agent. FIG. 3 illustratesanother alternate embodiment of conduit means 18 in which the conduit isarranged in an undulating, up and down, rather sinusoidal pattern inwhich the straight spans 66 of conduit are essentially vertical betweenthe connecting turns or curved portions 68.

FIG. 4 illustrates still another alternate embodiment of conduit means18 in which the conduit is arranged in a spiral whose radius decreasesbetween inlet 16 and outlet 36. An increasing radius from inlet tooutlet would provide more gentle turns for the growing clumps offlocculated solids. The axis of the spiral may be horizontal, verticalor at any intermediate orientation. The spiral may be open or flattened.In the embodiment of FIG. 4, the spent solution, precipitating agentsand flocculating agent flow upwardly in the conduit, which helps toimprove growth of the clumps of flocculated solids, particularly inintermittently operated systems. Upward flow also may be used in theother embodiments of conduit means 18.

In some applications, the orientation of the mixing and precipitatingportion of the coiled versions of conduit 18 advantageously may bedifferent from that of the flocculating portion of conduit 18. Forexample, improved mixing of the effluent and the precipitating agent maybe achieved if the portion of the coil for mixing the effluent and theprecipitating agent is positioned with the axis of the coil essentiallyhorizontal. The flocculating and clump-forming portion of the coil maybe positioned below the mixing and precipitating portion, with the axisof the flocculating and clump-forming portion essentially vertical. Forexample, in the embodiment of FIG. 4, the portion of the coil betweeninlets 27 and 33 would be positioned above the remainder of the coil andwould have an essentially horizontal axis. An example of thisarrangement will be discussed with regard to FIG. 34.

FIGS. 5 and 6 illustrate yet another alternate embodiment of conduitmeans 18 in which the conduit is coiled into a flat coil withconventional T-fittings 70 for connecting adjacent sections of conduitand delivering precipitating and flocculating agents to the conduit. Theembodiments of FIGS. 3 to 6 also may include static mixing elementslocated as in the embodiment of FIG. 2 and in Example 1. Those skilledin the art will appreciate that conduit means 18 may be formed into awide variety of regular or irregular geometric shapes including ovals,figure-eights, triangular or rectangular coils, flattened helixes andspirals and the like, without departing from the scope of the invention.

FIG. 7 illustrates another embodiment of the apparatus of the inventionin which conduit means 18 increases in flow area between inlet end 16and outlet end 36. The increase in flow area may be stepwise, asillustrated, or gradual without departing from the scope of theinvention. The increasing flow area permits the precipitated andflocculated solids to grow or ripen more fully as they moveprogressively more slowly along the conduit, thereby reducing thepercentage of fines delivered to collecting vessel 38. Such a reductionin the fines reaching the collecting vessel has been observed visuallyas the clumps of flocculated solids grow during movement along theconduit and as the percentage of silver-TMT fines decreases in theclarified liquid. Also, calculated residence times in the conduitcorrelate with reduction in fines; that is, in a continuous flow ofliquid and large, porous clumps in accordance with the invention, longerresidence times lead to reduced fines. An initial section 72 of conduitmeans 18 receives a mixture of spent solution and flocculating agent andpreferably includes static mixing elements 64.

EXAMPLE 2

For example, with a flow rate of spent solution in the range of 175 to400 ml/min and a flow rate of precipitating agent (TMT-15) in the rangeof 3 to 8 ml/min, tubing having an internal diameter in the range of0.250 to 0.375 inch (6.35 to 9.53 mm) and a length in the range of 0.5to 3.0 inch (12.7 to 76.2 mm) as was suitable for section 72. More than500 gallons (1893 L) of such spent solutions were processed. As themixture flowed along section 72, the spent solution and theprecipitating agent mixed thoroughly and particles of precipitate formedand grew. At the downstream end of section 72, the flocculating agent(American Cyanamid Magnifloc 846A) was delivered into the conduit atT-fitting 70 at a flow rate with range of 5 to 25 ml/min. The mixture ofliquid, precipitate, flocculant and flocculated solids then flowed intoa section 74 of conduit means 18 having an internal diameter in therange of 0.375 to 0.625 inch (9.53 to 15.88 mm) and a length in therange of 18 to 30 inches (457 to 762 mm). As the mixture flowed at lowerspeed along section 74, the precipitate gradually formed or ripened intolarger and larger clumps of flocculated solids. The mixture then flowedinto a section 76 of conduit means 18 having an internal diameter in therange of 0.75 to 1.0 inch (19.1 to 25.4 mm) and a length in the range of18 to 30 inch (457 to 762 mm). As the mixture flowed at still lowerspeed along section 76, the clumps of flocculated solids continued togrow. The mixture then flowed into an upwardly running section 78 ofconduit means 18 having an internal diameter in the range of 1.25 to1.75 inch (31.8 to 44.5 mm) and a length in the range of 74 to 78 inches(1880 to 1982 mm). As the mixture flowed even more slowly along section78, the clumps of flocculated solids continued to grow and reachedsubstantially their maximum size when the mixture reached outlet end 36.Suitable connectors 80 were used to join sections 74, 76 and 78. Fromoutlet end 36, the mixture of liquid and ripened flocculated solidsflowed through downcomer 48 into collecting vessel 38, where theflocculated solids agglomerated to form mass 50 and the entrained liquidrose toward the outlet of the collecting vessel. Total silver content inthe liquid effluent was in the range of 0.17 to 0.6 mg/L.

In each of sections 74, 76 and 78, the diameter and length should bechosen to facilitate formation of clumps of flocculated solids andpermit the largest porous clumps of flocculated solids to pass readilywithout being broken up too much. However, the diameter and geometry orlayout of each section should not be so constricted that the clumps maysettle in low spots during prolonged idle periods and block the flow orso large that the clumps do not substantially fill the cross section ofthe conduit and therefore might be easily bypassed by the liquidcarrying fines. Those skilled in the art will appreciate that suchdiameters will vary depending on factors such as the concentrations ofthe spent solution, precipitating agent and flocculating agent; and theflow rate through the conduit.

FIG. 8 illustrates a particular, more compact embodiment of the conduitmeans 18 of FIG. 7. In this case, the conduit may be formed readily as apanel of two sheets of thermoplastic which have been suitablythermoformed or from plate stock of suitable material which has beenconventionally machined, or blow molded, or injection molded, or formedby any convenient manufacturing process. For example, a pair of sheets82 of thermoplastic material such as polyvinyl chloride oracrylonitrile-butadiene-styrene may be thermoformed and sandwichedtogether to define conduit means 18. For ease of observation, one ofsuch sheets may be transparent or translucent. An initial mixing andripening section for spent solution and precipitating agent is definedbetween the inlet from check valve 26 and the inlet from check valve 32.

EXAMPLE 3

For infeed conditions of the type described for Example 2, the initialmixing section had an internal diameter of about 0.5 inch (12.7 mm) anda length of about 32 inches (813 mm). Depending on the particular spentsolution and precipitating agent, static mixers 64 could be provided inthis initial mixing section. Downstream of the inlet for flocculatingagent through valve 32, a second mixing and ripening section began withan internal diameter of about 0.5 inch (12.7 mm) over a length of about16 inches (407 mm); then extended through a smooth transition portion 84into a section having an internal diameter of about 0.875 inch (22.23mm) over a length of about 32 inches (813 mm); and finally extendedthrough a smooth transition portion 86 into a section having an internaldiameter of about 1.25 inch (31.8 mm) over a length of about 48 inches(1219 mm) terminating at a smooth portion 88 which opened through outletend 36. If desired, mounting pockets for check valves 20, 26 and 32could be incorporated between the thermoplastic sheets. In theembodiment of FIG. 8, the straight runs of conduit means 18 preferablywere arranged vertically in operation.

Based on laboratory tests, during operation porous clumps of flocculatedsolids formed and grew or ripened as they passed up and down through theconduit, with the clumps agglomerating to form progressively larger andlarger masses of flocculated solids while the liquid flow acted to drivethe mixture through the conduit. The growing masses of flocculatedsolids tended to fill the cross-sectional area of the conduit and toentrap fines moving along in the liquid. Total silver content in theliquid effluent was less than 1.0 mg/L, after the polishing filter.Those skilled in the art will appreciate that the conduit means for FIG.7 may be formed from a series of two or more connected panels ofconduits or by other conventional means of forming a plastic part,without departing from the scope of our invention. Parallel panels alsocould be used.

For some spent solutions containing silver in an amount less than about200 mg/L, addition of the precipitating agent may generate a highpercentage of fine particles which are relatively slow to agglomerateeven after addition of the flocculating agent. The modification of FIG.9 is configured to reduce the number of such fines. A furtherimprovement in fines reduction will be discussed with regard to FIG. 25.A generally cylindrical settling vessel 90 is provided for receiving theeffluent of flocculated solids, fines and liquid from conduit means 18.Vessel 90 comprises a sloped or concave bottom 92 having an outletopening 94 at its lowest point. A valve 96 optionally may be included tocontrol the flow of solids and liquids, as described in more detail withregard to FIGS. 21 to 29. Within vessel 90 a baffle plate or wall 98extends across a chord of the cross section of the vessel and terminatesat a lower edge 99 near to bottom 92. Solids and liquid flowing inconduit means 18 are discharged through outlet end 36 into the openupper end of an enclosed inlet passage or downcomer 100 on one side ofbaffle plate 98. Alternatively, a downcomer tube may be used rather thanbaffle plate 98. See also the discussion of the embodiment of FIG. 11.As fines, clumps of flocculated solids and liquid flow downward inpassage 100, the clumps continue to agglomerate and fines continue toadhere to each other and to existing clumps. As the clumps of solidsmove through valve 96, they essentially fill the cross sectional area ofthe valve and help to strain out fines. On the opposite side of baffleplate 98 near the upper end of vessel 90, an outlet opening 102 isprovided for clarified liquid rising within the vessel. At the bottom ofthe settling vessel, rather than forming a thick accumulation for laterremoval in the manner of the prior art, flocculated solids settlecontinuously or intermittently through normally open valve 96 intocollecting vessel 38 in the manner previously described. Thus, once theflocculated solids settle to the bottom of vessel 38, they essentiallyare not again disturbed. Liquid that enters collecting vessel 38 may beremoved in the manner described with regard to FIG. 1. Alternatively,the liquid simply may be displaced upward gradually within thecollecting vessel until the liquid actually flows slowly back intosettling vessel 90, in the opposite direction of the solids passingdownward through valve 96, and eventually leaves through outlet 102.When the collecting vessel is full, valve 96 is simply closed and vessel38 is removed and replaced without any need to disturb again the solidsalready settled therein.

Since a small amount of liquid and solids may remain in the short lengthof tubing below valve 96 but above collecting vessel 38, the arrangementof FIG. 10 may be used to provide a convenient way to avoid spilling. Anoutlet 104 for liquid is provided in or near the top of the collectingvessel and is connected via a suitable conduit 106, which optionally mayinclude a filter for fines, to an overflow collection vessel 108 forclarified liquid. Then, after valve 96 has been closed, outlet 104 maybe opened and connected to conduit 106 to allow the small amount ofliquid and solids to drain into the collecting vessel and displaceliquid through opening 104 to collection vessel 108. Vessel 38 may betilted slightly for decanting, if necessary. When the small amount hasdrained from below valve 96, outlet 104 may be closed and the liquid invessel 108 may be returned to settling vessel 90, for instance.

FIG. 11 illustrates an embodiment of the invention in which the flatreactor coil of FIGS. 5 and 6 has been combined with the settling vesselof the apparatus of FIG. 9. A standpipe or downcomer 110 may be attachedto conduit means 18 and extended downward through inlet passage 100toward bottom 92. Thus, flocculated solids and fines passing downwardthrough standpipe 110 will experience minimal additional shearing actionand will have additional time to grow or ripen before flowing out intothe still larger inlet passage 100 and on toward the bottom of settlingvessel 90. Because standpipe 110 ends well above the lower edge 99 ofbaffle 98, any buoyant particles leaving standpipe 110 will tend to risewithin passage 100 and have still more time to precipitate, agglomerateand settle. Thus, settling time for fine particles is extended in thisembodiment. At the same time, the clumps of flocculated solids willsettle to bottom 92 and move on through outlet opening 94 into thecollecting vessel.

FIG. 12 illustrates another embodiment in which sources 22 and 28 and aholding tank 111 for spent solutions respectively deliver precipitatingagents, flocculating agents and spent solutions into a rigid conduitmeans 18 extended downward into inlet passage 100 next to baffle 98. Ifdesired, conduit means 18 may be attached to or formed within baffle 98.For applications in which particularly effective flocculation isachieved within conduit means 18, baffle 98 may be omitted. Preferably,the conduit includes static mixer elements 64 upstream of the point ofdelivery of the flocculating agent. Depending on the degree of mixing ofspent solutions and precipitating agent needed to ensure adequateprecipitation, the length of conduit including optional static mixingelements can vary and flocculating agent from source 28 can be deliveredinto conduit means 18 at any point along a span 112 of the conduit. Ifnecessary to permit further growth of the clumps of flocculated solids,an extension conduit 114 can be added to lengthen conduit means 18. Apolishing filter 116 may be connected to outlet 102 if necessary forremoval of fines.

In operation of the embodiment of FIG. 12, spent solutions are pumpedout of holding tank 111 directly into conduit means 18. Precipitatingagent is pumped from source 22 into the conduit just after entry of thespent solution and the two are mixed vigorously by mixer elements 64.After sufficient mixing of these solutions has been achieved, theflocculating agent is introduced along span 112. No further mixerelements are used after introduction of the flocculating agent, to avoidbreaking up the clumps of flocculated solids as they move along conduitmeans 18. Those skilled in the art will appreciate that the precisepoint for introduction of the flocculating agent in this and otherembodiments of our invention can be determined empirically and willdepend on the number and type of mixer elements needed, the compositionsand concentrations of the spent solutions and the precipitating agents,the type and concentration of the flocculating agent, the viscosity ofthe overall mixture, the pressures and flow rates of the various liquidsand related factors. As the clumps of solids move through valve 96, theyessentially fill the cross sectional area of the valve and help tostrain out fines.

FIG. 13 illustrates still another embodiment of the invention in whichthe various solutions are mixed in a vessel rather than in a continuousconduit. A valve 118 and a valve 120, respectively, are used to controlgravity flow of precipitating agent and flocculating agent into a mixingvessel 122. Within holding tank 111, a float switch 124 is used toindicate the presence of a sufficient volume of spent solution towarrant operating the system. A valve 125 may be closed to stop flowfrom holding tank 111. Pump 14 delivers the spent solution into mixingvessel 122 where a float switch 126 is used to indicate the presence ofa sufficient volume of all three solutions. Mixing in vessel 122 mayresult simply from turbulence of the liquids as they are introduced intothe vessel or from use of a conventional prop mixer, not illustrated. Avalve 128 may be closed to prevent back flow to the inlet of pump 14. Avalve 130 may be closed to prevent flow toward a pump 132 used todeliver a mixture of liquids, precipitates and growing or ripeningclumps of flocculated solids into the upper end of conduit 110 withinsettling vessel 90. Pump 132 preferably is a bellows pump or similarpump with low shearing tendencies to minimize any tendency to break upclumps of flocculated solids. As the clumps move downward in conduit110, they continue to grow or ripen. A conventional programmablecontroller, not illustrated, may be used to govern to operation of thevarious pumps and valves.

The system of FIG. 13 preferably functions in a sort of continuous batchmode in which batches of liquids are pumped through periodically.However, those skilled in the art will appreciate that continuousoperation also can be achieved in accordance with the invention, exceptfor brief shutdown periods to exchange collecting vessels 38, to changefilters 116 or, if needed, to replenish sources 22 and 28. In acontinuous batch mode, spent solutions from photoprocessing systems areadded to holding tank 111, either by manual dumps or by hoses or pipesfrom the photoprocessing system. Valves 118, 120, 125, 128 and 130 areclosed. When the level of spent solutions reaches a predetermined level,float switch 124 signals the controller to open valve 125 and turn onpump 14 to deliver spent solution to mixing tank 122. As spent solutionis pumped into mixing vessel 122, valve 118 is opened by the controllerto deliver a predetermined quantity of precipitating agent which mixesturbulently with the incoming spent solution. When the level ofsolutions reaches a predetermined level, as determined by a timer in thecontroller or by float switch 126, the controller closes valve 125 tostop the flow of spent solutions from tank 111, opens valve 128 to drainvessel 122 to the inlet of pump 14 and closes valve 118 to stop thedelivery of precipitating agent. Pump 14 continues to operate, at thesame or a lower speed, to circulate the mixture of spent solution,precipitating agent and ripening precipitate from vessel 122 throughvalve 128 to pump 14 and back to vessel 122. After sufficient time haspassed for adequate mixing, valve 120 is opened by the controller todeliver a predetermined quantity of flocculating agent into mixingvessel 122 and then closed. Circulation of the mixture may continue fora short time at the same or lower speed until sufficient mixing hasoccurred to form a satisfactory flocculated precipitate. Pump 14 then isstopped; valve 128 is closed; valve 130 is opened; and pump 132 isturned on to deliver liquids, precipitates and flocculated solids to theinlet of conduit 110 within settling vessel 90. Alternatively, ratherthan circulating the mixture after addition of the flocculating agent,pump 14 may be stopped; valve 128 may be closed; valve 130 may beopened; and pump 132 may be turned on to deliver the mixture. Though twopumps 14 and 132 are illustrated, those skilled in the art willappreciate that these functions can be performed with a single pump andsuitable valving and piping.

Within conduit 110, the flocculated solids grow or ripen as they settletoward the bottom of vessel 90. At the lower end of baffle 98, most ofthe liquid and some of the fines separate from the clumps of flocculatedsolids and flow upward, eventually leaving through outlet 102 and filter116 where any remaining fines are removed. The liquid discharged fromfilter 116 may be discarded. At the same time, most of the flocculatedsolids flow downward through valve 96 into collecting vessel 38. As theclumps of solids move through valve 96, they essentially fill the crosssectional area of the valve and help to strain out fines. When vessel 38has filled with flocculated solids and some liquid, valve 96 is closedmanually to permit the full vessel to be removed and replaced with anempty one. Any liquid between valve 96 and collecting vessel may becollected manually and returned to settling vessel 90 or holding tank111. Vessel 38 of course may be removed and replaced at any time,whether full or not.

FIGS. 14 to 20, 30 and 31 illustrate alternative forms of collectingvessel 38 in accordance with the invention, which include various typesof built-in filters to remove fines from the clarified liquid. In eachof these collecting vessels, the liquid and clumps of agglomeratedsolids flow into a first chamber of larger flow area than that of theinlet conduit to the vessel, so that the flow velocity decreases and theclumps are permitted to settle, predominantly due to gravity effect, tothe bottom of the vessel, before the liquid encounters the filterelement. Thus, the life of the filter element is extended since most ofthe incoming solids do not encounter the filter before settling. Whenmetals are to be removed from the flocculated solid, all materials ofthe collecting vessel may be combustible, to enable a refiner to placethe entire container in a refining furnace unless it is an objective torecycle the containers for reuse.

These collecting vessels may be used for primary collection andfiltration as in the systems of FIGS. 1 to 8, where the flocculatedsolids are settled directly in the vessel without any presettling.Typically, this would mean that all liquid undergoing treatmenteventually would pass through the collecting vessel. Such an arrangementis appropriate where the flocculated or precipitated solids tend tosettle quickly; however, buoyant or pasty solids might tend to blind orclog the filter rather quickly. Alternatively, such collecting vesselscan be used for secondary settling and filtration, as in the systems ofFIGS. 9 to 13, where much of the clarified liquid is removed from thesettling vessel 90 and the solids are presettled in settling vessel 90before passing into collecting vessel 38. In such secondaryapplications, the outer from collecting vessel can be closed much of thetime, making settling the primary mechanism of separation of liquid andsolids, and can be opened only when necessary to remove accumulatedliquid. A tertiary use of such collecting vessels would be as receiversfor liquid flowing from settling vessel 90 through outlet 102 where asmall amount of residual fines may be present.

Except for the embodiments of FIGS. 30 and 31, each vessel 38 comprisesa simple pail or bucket 134 which preferably is partially or fullytransparent or translucent to permit visual observation of mass 50 offlocculated solids on the bottom of the vessel. The vessel may be closedby a preferably sealed lid or closure 136 so that, once filled, thevessel may be capped and used as a shipping container. In the embodimentof FIG. 14, an inlet conduit 138, which may be nested within anothersolid or perforated cylindrical baffle 139, extends downward from theunderside of closure 136 and preferably extends near to the bottom ofbucket 134, not illustrated. Thus, as previously described, flocculatedsolids will tend to settle around the lower end of the inlet conduit andforce incoming liquid and solids to flow into and through previouslysettled solids to remove fines and improve growth of the clumps offlocculated solids. Generally, the smaller the diameter of bucket 134,the more the solids will tend to stir up in the bottom as more liquidand flocculated solids are introduced. For flow rates into the bucket inthe range of 50 to 1000 ml/min, an internal diameter in the range of 4to 30 inches (102 to 762 mm) would be expected to give good results. Afilter support disk 140 extends radially around the inlet conduit andprovides a support for the upper edge of an annular filter element 142,such as a ring of pleated paper filter material. An annular filtersupport ring 144 supports the lower edge of filter element 142. Thus,liquid and any fines flowing upward through mass 50 of flocculatedsolids move toward filter element 142 where the fines are removed. Theclarified liquid then passes through a radial and axial clearance 146surrounding the filter and above support disk 140, and leaves the vesselthrough a top outlet 148.

In the embodiment of FIG. 15, filter element 142 is replaced by a bagfilter 150 of suitable porosity. Preferably, inlet conduit 138 extendsclose to the bottom of the bucket, not illustrated. The edge of themouth of filter 150 is supported at the periphery of support disk 140.The clarified liquid then flows through the bag filter and leavesthrough a top outlet 152. The bag filter also helps to wick liquid outof mass 50. When the filter is full, vessel 38 is removed and replaced.In the embodiment of FIG. 16, support disk 140 has been replaced by anannular filter support ring 154 mounted on the underside of closure 136;and a side outlet 156 is provided through the wall of the bucket.

In the embodiments of FIGS. 17 to 20, conventional cylindrical, pleatedpaper filter cartridges are used to remove fines. In the embodiment ofFIG. 17, the axis of a filter cartridge 158 is set horizontally to allowthe clarified liquid to pass from the interior of the filter throughoutlet 156. In the embodiment of FIG. 18, inlet conduit 138 extendstoward the bottom of the bucket at an off-center location. A baffle wall160 extends across the width of the bucket to stop solids from shortcircuiting to the filter, thus allowing fines more time to agglomerateand clumps of solids more time to settle into mass 50. A filtercartridge 162 is set horizontally on the opposite side of baffle wall160 from inlet conduit 138, to allow the clarified liquid to passthrough a centrally located top outlet 164. In the embodiment of FIG.19, the positions of inlet conduit 138 and outlet 164 are reversed fromthose of FIG. 17; and a filter cartridge 166 is set vertically. In theembodiment of FIG. 20, the positions of inlet conduit 138 and outlet 164are reversed from those of FIG. 18. A restriction may be provided inoutlets 156 and 164 to control the flow rate through collecting vessel38 and reduce the chances of stirring up fines from mass 50.

Normal density differences between the flocculated solids and the liquidshould allow the solids to settle into vessel 38 and the liquids to riseback into settling vessel 90 for discharge through outlet 102 or to risewithin vessel 38 for discharge through outlet 56, 148, 152, 156 or 164.When the collecting vessel first begins to fill, entry of solids andliquid naturally will cause convection currents, coupled with Brownianand displacement movement of the solids. In the collecting vessels ofFIGS. 1, 14 to 20, 30 and 31, these currents and disturbances areconfined to the interior of the vessel. If the liquids and solids aredischarged from conduit 138 a substantial distance above the bottom ofthe vessel, the height through which the solids must settle willdecrease as the vessel fills and mass 50 accumulates, which will tend toreduce such currents and disturbances. However,as previously discussed,preferably the flocculated solids and liquids are discharged near thebottom of the collecting vessel to take advantage of the filteringeffect of mass 50. For the collecting vessels of FIGS. 1 and 14 to 20,the vessels eventually will be substantially full of settled solids,plus a rather thin covering layer of liquid and some liquid heldinterstitially within mass 50. At that time, the full vessel is removedand replaced.

FIGS. 21 to 29 illustrate various alternative features of the inventionwhich can be used to connect and disconnect vessel 38 from the overallsystem. Whether or not vessel 38 includes an outlet for liquid, a quickdisconnect fitting, such as a conventional Banjo fitting, may beprovided in the conduit leading into vessel 38. Suitable conventionalquick disconnect fittings are available from Terra Products, Inc. ofCrawfordsville, Ind. The female end 168 of such a fitting cooperates inthe known manner with the male end 170 and the fitting may be orientedas illustrated in FIG. 21 or FIG. 22. In the arrangement of FIG. 22,liquid and solids draining from male end 170 tend to flow into the opencavity 176 of female end 168, thus reducing chances of spillage. Belowthe fitting, a standpipe 172 extends upward from a screw-on cap 174suitably mounted to vessel 38. Thus, when the flow of liquids and solidsis stopped, such as by closing valve 96, and fitting 168/170 isdisconnected, a small mount of liquids and solids will remain instandpipe 172. Vessel 38 simply may be tipped to pour off this smallmount, after which cap 174 and standpipe 172 are removed and replacedwith a plain screw on cap. Cap 174 and standpipe 172 are then mounted toan empty collection vessel and fitting 168/170 is reconnected to permitcontinued operation of the system. If the flocculated solids have arather mucoid consistency, such as flocculated silver TMT precipitate,the flow areas through the valve and quick disconnect fitting and intovessel 38 must be large enough to permit solids to move downward andliquid to move upward, such as in the embodiments of FIGS. 12 and 13.For example, a flow area approximately 0.75 to 3.0 inch (19.1 to 76.2mm) in diameter has been found to be effective for flocculated silverTMT precipitate. To aid with drainage into vessel 38, valve 96 may be athree-way valve 178 of the general type illustrated in FIGS. 23 and 24.A vent conduit 180 is connected to one port of valve 178; so that, withthe valve closed as in FIG. 24, liquid and solids flow readily downwardthrough the quick disconnect fitting 168/170.

Where the density of the flocculated solids is close to that of theremaining liquids and there is a tendency for fines to be generated,operation of collecting vessel 38 can be improved by providing one pathinto the vessel for downward moving flocculated solids and liquids andanother, separate path from the vessel for clarified liquids. Such anarrangement also helps to prevent fines from short circuiting thecollecting vessel. FIGS. 25 to 29 illustrate various embodiments of suchseparate paths.

In the embodiment of FIG. 25, an extension conduit 182 is provided fromthe outlet end 36 of conduit means 18 downward in settling vessel 90,through outlet 94 and into collecting vessel 38. Due to thisarrangement, clarified liquids rising from vessel 38 can pass upwardthrough opening 94 without disturbing or being disturbed by the downwardflow in conduit 182. A baffle plate 184 extends across settling vessel90 opposite outlet 102; so that, a portion of any fines rising into thesettling vessel will have an opportunity to agglomerate and settledownwardly through outlet 94. A potential drawback of the embodiment ofFIG. 25 is the lack of a valve between the settling vessel and thecollecting vessel, which can complicate removal and replacement of thecollecting vessel. One alternative arrangement which eliminates thisdrawback is illustrated in FIG. 26. Here, in parallel with standpipe172, vessel 38 is provided with a top outlet 186 which is connected to avalve 188 by a suitable conduit 190. Thus, clarified liquid can bewithdrawn through conduit 190 and returned to settling vessel 90, if asettling vessel is used, or discharged from the system.

Another alternative arrangement is illustrated in FIG. 27. The inletconduit 48 for flocculated solids and liquid extends into vessel 38through a fitting 192 which positions conduit 48 concentrically withinan outlet conduit 194 for clarified liquid. The flow area of conduit 48preferably is considerably larger than that of conduit 194, to alloweasy passage of clumps of flocculated solids. Fitting 192 preferably isthreaded for removal from vessel 38. Valve 188 functions as in theembodiment of FIG. 26. Quick disconnect fittings would be provided belowboth of valves 96 and 188. FIG. 28 illustrates a variation of theembodiment of FIG. 27 in which the two valves share a common actuator196. In the embodiments of FIGS. 26 and 27, the inlet and outletconduits could be arranged side by side rather than concentrically. FIG.29 illustrates another alternative arrangement in which a combined inletand outlet conduit 198 comprises a central divider wall 200 to definesuch side by side conduits. A ball valve 202 is provided with a dividedflow passage for simultaneously opening and closing the two conduits.

FIG. 30 illustrates still another embodiment of collecting vessel 38which is particularly useful in accordance with the invention. Acylinder 210, preferably transparent or translucent and preferably butnot necessarily circular in cross section, is made from any suitablematerial such as clear plastic. A top end cap 212 and a bottom end cap214 are provided with central bosses 216, 218 which extend into theinterior of cylinder 210 and support between them a conventionaltubular, pleated paper filter element 220. The filter element preferablyshould be a single pass filter made from totally combustible materials,should have a nominal porosity less than 0.5 microns and should havesufficient structural rigidity to withstand operating pressuredifferentials. Although filter element 220 as illustrated extends frombottom end cap 214 to top end cap 212, a shorter filter element whichterminates below top end cap 212, or above bottom end cap 214, or both,may also be used without departing from the invention. An upwardlyextending annular collecting chamber 222 is separated by filter element220 from an upwardly extending liquid discharge chamber 224 definedwithin the filter element. Although a tubular pleated filter element ispreferred to define chambers 222 and 224, those skilled in the art willappreciate that any filter element could be used, such as a flat filterextended across a chord of cylinder 210, which divides the interior ofthe cylinder into parallel, upwardly extending collecting and dischargechambers. At the bottom of chamber 222, an inlet 226 is provided forflocculated solids and liquid; however, a suitable downcomer conduitcould also be used to introduce the solids near the bottom of chamber222, in a manner similar to conduit 48 of the embodiment of FIG. 1. Thesolids and liquid could also be introduced through the side wall ofcylinder 210 near the bottom of chamber 222. At the bottom of chamber224, an outlet 228 is provided for clarified liquid which has passedthrough filter element 220. In one actual embodiment of this collectingvessel, cylinder 210 had an inside diameter of about 6.0 inches (152 mm)and a length of about 20 inches (508 mm). Filter element 220 wasassembled from a pair of commercially available Harmsco filter elementsNo. 801-0.35, manufactured by Harmsco, Inc. of North Palm Beach, Fla.The filter elements were placed end-to-end and had a nominal porosity ofabout 0.35 microns, an inner diameter of about 1.0 inch (25.4 mm) and anouter diameter of about 2.375 inch (60.33 mm).

During use of the collecting vessel of FIG. 30, clumps of flocculatedsolids accumulate on the bottom of chamber 222 to form mass 50 andliquid flows through filter element 220 to outlet 228. As mass 50 risesaround filter element 220, the lower portion of the filter elementgradually becomes partially obstructed due to the presence of mass 50.However, the liquid which rises out of mass 50 continues to flow througha fresh or relatively unobstructed portion of the filter element. Asfilter element 220 becomes more less blocked by the progressive rise ofmass 50, the effective outlet from chamber 222 moves upward along thefilter element. Thus, even when the collecting vessel essentially isfull of flocculated solids, there continues to be a percentage of thefilter through which the liquid can pass. Since the liquid and solidsare introduced near the bottom of chamber 222, many fines tend to befiltered out within mass 50, which further reduces the burden on thefilter element, though increased inlet pressure may be needed to forcethe liquid and solids into mass 50 at the end of a run.

FIG. 31 illustrates an alternative embodiment of the collecting vesselof FIG. 30. Cylinder 210 and end caps 212, 214 have been replaced by amolded shell or housing 211 having domed ends 213a, 213b rather like acommon container for carbonated beverages. The domed ends permitpressurization of the container without much concern for loosening orleaking of end caps. A closure plug 219 extends upwardly into housing211 to support filter element 220. A suitable threaded connection 221secures plug 219 to housing 211. Inlet 226 and outlet 228 extend throughplug 219. Alternatively, inlet 226 may be provided through the wall ofhousing 211 near the bottom of chamber 222, not illustrated. Suitableplugs, not illustrated, are used to close inlet 226 and outlet 228 forshipment of a full collecting vessel. In the collecting vessels of FIGS.30 and 31, chamber 222 may be provided with a one way vent valve, notillustrated, to admit air to the chamber to prevent formation of avacuum and to facilitate drainage through outlet 228.

FIG. 32 illustrates schematically an apparatus or system according tothe invention which comprises the collecting vessels of FIG. 30 or 31.Many of the components of the embodiment of FIG. 1 are included. Holdingtank 111 is provided with a low level shut-off float switch 230 and ahigh level shut-off float switch 232. Low level switch 230 is positionedto provide a control signal when the volume of spent solutions falls toa residual volume needed for blending with subsequently added spentsolutions to damp out differences in composition and concentration ofspent solutions introduced into conduit means 18. Those skilled in theart will appreciate that a holding tank embodying such a low levelswitch and residual volume also could be used for delivery of spentsolutions in the embodiments of FIGS. 1 to 13. The discharge ofprecipitating agent from pump 24 passes into conduit means 18 at inlet27 just upstream of an inlet end of a section of conduit means 18comprising optional static mixer elements 64. The discharge offlocculating agent from pump 30 passes into conduit means 18 at inlet 33just upstream of an inlet end of a conditioning coil 242 of suitabletubing, in which the clumps of flocculated solids continue to grow orripen before entering collecting vessel 38. As in previously describedembodiments, many enlarged clumps individually would extend across andsubstantially fill the cross-sectional area of coil 242, to produce thedesired blocking and straining effect for remaining fines. If desired, astatic mixing zone also may be included between inlet 33 andconditioning coil 242. A pressure gage 244 near the inlet to collectingvessel 38 indicates the inlet pressure, which would be expected to riseas vessel 38 fills with flocculated solids. A pressure switch 246 sensesthis same pressure and signals a conventional programmable controller248 when the inlet pressure exceeds a predetermined limit, at whichpoint the controller shuts off motor 34 to stop the various pumps; andcollecting vessel 38 is replaced. When the control signal from switch230 indicates a low level in holding tank 111, controller 248 shuts offmotor 34 until switch 232 signals that a sufficient volume of spentsolutions has accumulated for continued treatment.

In one actual version of the system illustrated in FIG. 32, holding tank111 had a volume of about 20 gallons (75.71 L). Conduit means 18 wasformed from 0.25 inch (6.35 mm) internal diameter tubing upstream ofconditioning coil 242, which was formed from 30 feet (9.14 m) of 0.5inch (12.7 mm) internal diameter tubing. The section of static mixerswas about 2 inches (51 mm) long. The tubing from sources 22 and 28 hadan internal diameter of about 0.0625 to 0.125 inch (1.59 to 3.18 mm).Pumps 14, 24 and 30 were peristaltic pumps operated to provide a flowfrom holding tank 111 of about 200 ml/min. and flows from sources 22 and28 of about 4 ml/min. When a sufficient volume of spent solutions wasaccumulated in holding tank 111, float switch 232 signaled controller248 to start motor 34 and pumps 14, 24 and 30. Until the level in tank111 reached float switch 230, the system continued to operate.Well-grown or ripened clumps of flocculated solids were delivered fromconditioning coil 242 to the inlet of collecting vessel 38 andacceptably clarified liquid was discharged from the outlet of thevessel. The desired blocking and straining effect for remaining fineswas observed in the conditioning coil and the collecting vessel. Undercontinuous operating conditions, a collecting vessel 38 of the type andsize described with regard to FIG. 30 required replacement about every40 hours of continuous operation. When float switch 230 indicated lowlevel in tank 111 or sensor switch 246 indicated high inlet pressure tovessel 38, controller 248 stopped motor 34.

EXAMPLE 4

The spent solution contained paper process developer, paper processbleach-fix, paper process stabilizer, film process developer, filmprocess bleach, film process fix, and film process stabilizer. Initialsilver content of this mix of solutions was 1.7 g/L. The percentages ofeach of the spent solutions was in proportion to what would be expectedin normal operation of a typical minilab photoprocessor. This mix ofsolutions was treated in a system similar to those described in Example1 and shown in FIG. 32. The performance of the system was acceptable,but the flocculated solids were not as tightly bound as those seen inExample 1. The more loosely bound solids filled the settling filter,similar to that of FIG. 30, more quickly. The experiment was terminatedafter approximately 40 gallons (151.4 L) of solution were treated.Silver removal was excellent, with total (soluble plus insoluble) silvercontent of the system effluent analyzed to be in the range of 0.06 to0.3 mg/L.

EXAMPLE 5

The following minilab solutions were treated in a system similar tothose described in Example 1 and shown in FIG. 32. A settling filter asshown in FIG. 31 was used to separate the flocculated solids fromliquid. The minilab mix comprised: paper process bleach fix, simulatedpaper process stabilizer, film process fix, and film process stabilizer.Silver content of this mixture was 2.5-3.0 g/L. The flow rates were: 5ml/min of TMT-15, 20 ml/minute of a 400 PPM solution of Calgon 2406cationic polymer, and 200 ml/min of silver bearing solution. Filterutilization was excellent, with over 100 gallons (379 L) of solutiontreated before pressure build up signaled the need for a filter change.Silver removal was excellent for this experiment, which was repeatednumerous times to prove reliability. Total silver in effluent rangedfrom 0.2 to 0.96 mg/L.

FIG. 33 illustrates an alternative embodiment of the invention in whichthe spent solutions from conduit 12 and the appropriate mount ofprecipitating agent from source 22 are combined in a mixing vessel 250using a propeller mixer 252 driven by a motor 254. A pump 256 delivers amixture of liquid and precipitate along a conduit 258 to a point atwhich flocculating agent is delivered from source 28 upstream ofconditioning coil 242. Otherwise, this embodiment is much the same asthat of FIG. 32.

EXAMPLE 6

Five gallons (18.93 L) of minilab solutions were treated after primaryelectrolytic silver recovery: paper process bleach-fix, paper processstabilizer, film process fix, and film process stabilizer. The silvercontent after electrolytic silver recovery was measured at 220 mg/L. Thesolution was treated in the apparatus of FIG. 33. The TMT-15precipitating agent (25 ml) was added in proportion to the silver insolution, with continuous mixing in the reaction tank. A low molecularweight, cationic polymer, Calgon E-2280, made up at 400 mg/L in water,was injected at 20 ml/min. Silver removal from solution was good withthis arrangement, but the pleated paper filter element 220 in thesettling vessel blinded more quickly than expected. Although theeffluents from this system were within regulatory limits for silver, thesystem would have been somewhat more expensive to operate due likelyneed to replace filters more frequently. The flocculated solids weregranular, with some fines passing through the conduit 18. Evidence ofthis was a yellow color on the wetted filter element parts, and anincreasing liquid level inside the settling filter. Filter capacity wasnot determined for this mix of solutions.

EXAMPLE 7

A simulated combined minilab effluent containing paper processbleach-fix, paper process stabilizer, film process rut, and film processstabilizer, containing 2.5 to 3.0 g/L silver was used as the feedsolution in the apparatus shown in FIG. 33. Them were 2 reactors in thiscase. One reactor was tank 250 in which 400 ml of TMT-15 were added bypump 24 to 5 gallons (18.93 L) of the silver-bearing solution with goodmixing provided by a laboratory-scale propeller mixer 254 mounted on thetop of the reactor. The mixer speed was held at 100-200 rpm for theduration of the run. After 5-10 minutes reaction time, this slurry wasintroduced into a conduit 258 using a bellows pump 256 at 200 ml/minute.A 400 mg/L solution of Calgon Flocculant Product No. POL-E-Z-2406 wasinjected into conduit 258 using bellows pump 30 through a T-fittingsized to produce turbulence at the point of injection. From there, theslurry was carried into conditioning coil 242, which was made fromapproximately 30 feet (91.44 m) of 0.5 inch (12.7 mm) nominal diameterflexible polyvinyl chloride tubing wrapped around a plastic cylinderapproximately 12 inches (305 mm) in diameter. Flow was directed up thespiral in order to displace air bubbles and cause precipitated fines toencounter the growing or ripening clumps of flocculated solids. Fromcoil 242 the slurry was directed to the settling vessel of FIG. 30.Clarified filtrate was directed to the drain from the settling vessel.Filter capacity was determined by monitoring the pressure gage 244 builtinto the system. Total (soluble plus insoluble) silver levels leavingthe settling filter were analyzed at 0.3 to 0.7 mg/L using atomicabsorption silver analyses. More than thirty-five, five-gallon (18.93 L)batches of silver-bearing solution as described above were treated inthis apparatus before the back pressure in the settling filter reached1012 psig (68.9-82.7 kPa), indicating the need to change filters.

EXAMPLE 8

The following minilab solutions were treated in a system similar tothose described in Example 1 and shown in FIG. 32. A settling filter asshown in FIG. 31 was used to separate the flocculated solids fromliquid. The minilab mix comprised: KODAK Process RA-4 bleach fix pluslow flow wash which had been electrolytically desilvered. Silver contentof this mixture was 180 mg/L. TMT-15 solution was diluted 1:10 withwater. The flow rates were: 5 ml/min of diluted TMT-15, 20 ml/minute ofa 400 mg/L solution of Calgon 2280 cationic polymer, and 200 ml/min ofthe silver bearing solution. Although the flocculated precipitatedemonstrated a sandy or grainy appearance, there were no noticeablefines. Settling and subsequent filtration in the filter of FIG. 31appeared normal. Silver analysis by atomic absorption of the filtratewas in the range of 0.6 to 0.9 mg/L.

FIG. 34 illustrates a large scale apparatus according to the invention.A collection tank 260, which for example may have a capacity of about1,000 to 1,200 liters of spent photoprocessing solution, drains througha conduit 262 to a pump 264 which delivers the solution to an inlet endof an initial mixing conduit formed into a coil 266 having anessentially horizontal axis. For example, tubing with an internaldiameter of 25.4 mm and a length of about 2 to 4 meters is suitable forcoil 266 for flow rates of 1 to 3 L/min. A tank 268 of precipitatingagent and a tank 270 of flocculating agent drain through respectiveconduits 272, 274 to a multi-head pump 276. Precipitating agent isdelivered from pump 276 into a conduit 278 which empties into adownwardly flowing portion of coil 266 in the manner previouslydescribed. Flocculating agent is delivered from pump 276 through conduit280 which empties into a downward flowing conduit 282 which delivers themixture from coil 266 to an inlet at the bottom of a second conditioningconduit formed into a coil 284 having an essentially vertical axis. Forexample, tubing with an internal diameter of 25.4 mm and a length ofabout 10 to 12 meters is suitable for coil 284. The mixture flows upwardthrough coil 284 and exits the coil into a conduit 286 which emptiesinto a large collecting and shipping vessel 290, which for example mayhave a volume of about 1,000 to 1,200 liters. Preferably, conduit 286extends to the bottom of vessel 290, for reasons previously discussed.Liquid flowing from near the top of vessel 290 passes along a conduit292 to a final filter 294. The apparatus operates in essentially thesame manner of that of FIG. 1, for example.

Apparatus of the types described in this specification have been shownto be effective for removing silver from spent photographic processingsolutions having silver concentrations in a range of 180 mg/L to 8 gm/L,preferably about 2 to 4 gm/L. Removal was best in flow regimes havingNR_(Re) as high as about 4,000, corresponding to spent solution flowrates in a range of about 25 to 8,000 ml/min. Beyond N_(Re) of about4,000, formation of large, porous clumps of precipitate in the mannerpreviously described, was not observed; an abundance of fines wasobserved in the discharge from the apparatus; and final silver contentexceeded the regulatory requirements previously mentioned. For N_(Re)beyond about 4,000, any large clumps which might form momentarily wouldbe expected to break up quickly into small particles, due to the highflow rates. Preferably, the pH of the solution was in the range of 4 to12; and the temperature was in a range of 0 to 71 degrees Celsius.Conduit sizes in a range of 6.4 to 63.5 mm internal diameter wereeffective to ensure formation of porous clumps which essentially filledthe cross sectional area of the conduit, in the flow regimes justdescribed. When conduit 18 was formed into a spiral coil, 6.4 to 12.7 mminternal diameter conduit was effective for coils with a coil diameterof about 152.4 mm, while 25.4 mm internal diameter conduit worked wellwith a coil diameter of about 305 mm or greater. Silver content of thedischarge from the apparatus according to the invention was in a rangeof 1.4 to 8.6 mg/L, well below the regulatory requirements.

Those skilled in the an will appreciate from the foregoing descriptionand examples that the apparatus and methods of our invention may be usedto remove metals other than silver from other industrial spent solutionssuch as electroplating solutions, metal etching solutions and the like.For example, a rinse water from a catalyst-making process containing 200mg/L of nickel was treated in an apparatus of the type shown in FIG. 33by adding sodium hydroxide as a precipitating agent and CalgonPOL-E-Z-2406 as a flocculating agent. Clumps of agglomerated particlesof flocculated precipitate were readily formed and collected. Thoseskilled in the an will further appreciate that the apparatus and methodof our invention can be used readily to remove other metal species (suchas iron, copper, cadmium, lead, mercury, chromium, barium and aluminum)by precipitation using known precipitating agents (such as TMT,hydroxides, sulfides, sulfates or organic thiols), by flocculation usingknown flocculating agents (such as those mentioned in thisspecification) and by collection in one of our collecting vessels.Persons skilled in the an also will appreciate that the method andapparatus of our invention can be used to remove other, non-metallicspecies, including organic and inorganic compounds and materials such ashexacyanoferrates, sulfates, sulfides, phosphates, carbonates,photographic coupling agents, sewage sludge micro-organisms and thelike, using precipitating agents such as iron, calcium, carbon dioxideor similar agents, followed by an appropriate flocculating agent.Suitable examples of precipitating and flocculating agents are given inthe Bober and Cooley article and the Spears and Sentell articlepreviously mentioned, both of which are incorporated by reference intothis specification.

The method of removing silver-TMT precipitate from solution usingcationic polymers as flocculating agents, such as Product Nos.POL-E-Z-2406and E-2280 from the Calgon Corporation, is a separateinvention of our colleague A. Richard Szembrot, which is implemented bythe apparatus and method of our invention.

    ______________________________________                                        Parts List                                                                    ______________________________________                                                          58 . . . final filter                                       10 . . . apparatus for removing silver                                                          60 . . . straight span                                      12 . . . infeed conduit                                                                         62 . . . connecting turn                                    14 . . . pump for spent solutions                                                               64 . . . mixing elements                                    16 . . . inlet end of 18                                                                        70 . . . T-fitting                                          18 . . . reaction conduit                                                                       72 . . . conduit section                                    20 . . . check valve                                                                            74 . . . conduit section                                    22 . . . source of precipitating agent                                                          76 . . . conduit section                                    24 . . . pump for precipitating agent                                                           78 . . . conduit section                                    26 . . . check valve                                                                            80 . . . connector                                          27 . . . inlet to 18                                                                            82 . . . sheets of thermoplastic                            28 . . . source of flocculating agent                                                           84 . . . transition conduit portion                         30 . . . pump for flocculating agent                                                            86 . . . transition conduit portion                         32 . . . check valve                                                                            88 . . . smooth conduit portion                             33 . . . inlet to 18                                                                            90 . . . cylindrical settling vessel                        34 . . . motor for 14, 24, 30                                                                   92 . . . sloped or concave bottom                           36 . . . outlet end of 18                                                                       94 . . . bottom outlet from 90                              38 . . . collecting and shipping vessel                                                         96 . . . valve                                              40 . . . bottom of 38                                                                           98 . . . baffle plate or wall                               44 . . . top of 38                                                                              100 . . . inlet passage or                                                    downcomer                                                   46 . . . closure  102 . . . top outlet from 90                                48 . . . downcomer conduit                                                                      104 . . . outlet for liquid                                 50 . . . mass of flocculated solids                                                             106 . . . conduit                                           52 . . . separated liquid                                                                       108 . . . overflow collecting vessel                        54 . . . check valve                                                                            110 . . . standpipe or downcomer                            56 . . . discharge conduit                                                                      112 . . . span of 110                                       114 . . . extension of 110                                                                      184 . . . baffle plate                                      116 . . . polishing filter                                                                      186 . . . top outlet                                        118 . . . valve   188 . . . valve                                             120 . . . valve   190 . . . conduit                                           122 . . . mixing tank                                                                           192 . . . fitting                                           124 . . . float switch                                                                          194 . . . outlet conduit                                    125 . . . valve   196 . . . valve actuator                                    128 . . . valve   198 . . . inlet/outlet conduit                              130 . . . valve   200 . . . divider wall                                      132 . . . pump    202 . . . ball valve                                        134 . . . pail or bucket                                                                        210 . . . cylinder                                          136 . . . closure 211 . . . shell or housing                                  138 . . . inlet conduit                                                                         212 . . . top end cap                                       139 . . . baffle  213a, 213b . . . domed ends                                 140 . . . filter support disk                                                                   214 . . . bottom end cap                                    142 . . . annular filter element                                                                216, 218 . . . central boss                                 144 . . . filter support ring                                                                   219 . . . closure plug                                      146 . . . radial and axial clearance                                                            220 . . . filter element                                    148 . . . top outlet                                                                            222 . . . collecting chamber                                150 . . . bag filter                                                                            224 . . . discharge chamber                                 152 . . . top outlet                                                                            226 . . . inlet to 222                                      154 . . . filter support ring                                                                   228 . . . outlet from 224                                   156 . . . side outlet                                                                           230 . . . float switch                                      158 . . . filter cartridge                                                                      232 . . . float switch                                      160 . . . baffle wall                                                                           242 . . . conditioning coil                                 162 . . . filter cartridge                                                                      244 . . . pressure gage                                     164 . . . central top outlet                                                                    246 . . . pressure switch                                   166 . . . filter cartridge                                                                      248 . . . programmable controller                           168 . . . female end of fitting                                                                 250 . . . mixing vessel                                     170 . . . male end of fitting                                                                   252 . . . propeller mixer                                   172 . . . standpipe                                                                             254 . . . motor                                             174 . . . screw-on cap                                                                          256 . . . pump                                              176 . . . open cavity                                                                           260 . . . collection tank                                   178 . . . three-way valve                                                                       262 . . . conduit                                           180 . . . vent conduit                                                                          264 . . . pump                                              182 . . . extension conduit                                                                     266 . . . initial coil with horizontal                                        axis                                                        268 . . . tank for precipitating agent                                                          282 . . . down conduit                                      270 . . . tank for flocculating agent                                                           284 . . . second coil with vertical                                           axis                                                        272 . . . conduit 286 . . . conduit                                           274 . . . conduit 290 . . . collecting and shipping                                             vessel                                                      276 . . . pump    292 . . . conduit                                           278 . . . conduit 294 . . . final filter                                      280 . . . conduit                                                             ______________________________________                                    

While our invention has been shown and described with reference toparticular embodiments thereof, those skilled in the art will understandthat other variations in form and detail may be made without departingfrom the scope and spirit of our invention.

Having thus described our invention in sufficient detail to enable thoseskilled in the art to make and use it, we claim as new and desire tosecure Letters Patent for:
 1. Apparatus for removing silver from spentphotoprocessing solution, comprising:means for providing a spentphotoprocessing solution containing silver to be removed; conduit meansfor receiving and passing the spent photoprocessing solution, theconduit means defining a mixing path and having a cross sectional flowarea, an inlet end and an outlet end; first means for delivering thespent photoprocessing solution at a first flow rate into the inlet end;a source of precipitating agent for silver; second means for deliveringthe precipitating agent at a second flow rate into the conduit means; asource of flocculating agent for a precipitate formed by mixing thesolution and the precipitating agent; third means, located downstream ofthe second means by a first distance providing a first residence timesufficient for mixing of the solution and the precipitating agent andfor forming a silver precipitate for flocculation, for delivering theflocculating agent at a third flow rate into the conduit means; theoutlet end of the conduit means being located downstream of the thirdmeans for delivering by a second distance, such that the first, second,and third flow rates, the cross sectional flow area, and the seconddistance provide a second residence time sufficient for forming clumpsof flocculated particles of the silver precipitate; the second residencetime, the precipitating agent and the flocculating agent being such thatthe clumps of flocculated particles grow and agglomerate during movementof the solution and clumps through the second distance into enlargedclumps many of which individually extend across and substantially fillthe cross sectional flow area, whereby individual enlarged, movingclumps substantially block passage of and strain from the solutionremaining fine particles which then adhere to the enlarged, movingclumps as the enlarged clumps move to the outlet end; and a settlingvessel for receiving clumps and solution from the outlet end andallowing the clumps to settle from the solution.
 2. Apparatus accordingto claim 1, wherein the setting vessel has a sloped bottom wall and abottom outlet for liquid and flocculated solids.
 3. Apparatus accordingto claim 2, wherein the settling vessel has a cross section andcomprises an internal baffle wail extended across a chord of the crosssection, the baffle wall having a lower edge near the sloped bottomwall, the conduit means extending into the settling vessel on one sideof the baffle wall; and a second outlet for clarified liquid on anopposite side of the baffle wall near an upper end of the settlingvessel, whereby liquid and flocculated solids are discharged into thesettling vessel near the sloped bottom wall to allow flocculated solidsto settle through the bottom outlet and liquid to flow through thesecond outlet.
 4. Apparatus according to claim 1, wherein the settlingvessel comprises:a collecting and shipping vessel having an inlet,releasably connected to the outlet end of the conduit means, forreceiving the flocculated solids and any remaining liquid, forpermitting the flocculated solids to settle to a bottom of the vesseland the remaining liquid to move toward an outlet of the vessel, wherebythe settled flocculated solids gradually will agglomerate and compact tofill substantially the vessel while at least a substantial part of theremaining liquid will rise to an increasingly higher level andeventually pass through the outlet of the vessel, thereby permitting afilled collecting and shipping vessel to be disconnected from the outletend of the conduit means and replaced with an empty collecting andshipping vessel.
 5. Apparatus according to claim 4, wherein the outletof the collecting and shipping vessel is above the bottom of the vessel;and the flocculated solids and any remaining liquid flow into thecollecting vessel near the bottom of the vessel, whereby any remainingliquid flows upward through previously settled solids, thereby removingfines before the liquid reaches the outlet of the vessel.
 6. Apparatusaccording to claim 4, wherein the collecting and shipping vesselcomprises a filter for removing fines from liquid flowing through theoutlet of the vessel.
 7. Apparatus according to claim 6, wherein thefilter is a porous bag suspended within the collecting and shippingvessel, the flocculated solids being captured within the bag. 8.Apparatus according to claim 6, wherein the outlet from the collectingand shipping vessel is located above the bottom of the vessel; thefilter comprises a ring of pleated filter material supported at thelevel of the outlet from the vessel; and the inlet of the vessel opensinside the ring, whereby flocculated solids settle to the bottom of thevessel and remaining liquid eventually rises to flow through the ring tothe outlet of the vessel.
 9. Apparatus according to claim 4, wherein theflocculated solids and any remaining liquid flow into the collecting andshipping vessel near the bottom of the vessel, whereby any remainingliquid must flow upward through previously settled solids, therebyremoving fines from the liquid; a filter element is positioned withinthe vessel, the filter element dividing the interior of vessel into afirst chamber for receiving flocculated solids and liquid and a secondchamber for receiving liquid passed through the filter element; and theoutlet of the vessel is located at the bottom of the vessel in thesecond chamber.
 10. Apparatus according to claim 9, wherein the filterelement extends upward from the bottom of the collecting and shippingvessel and is tubular; and the second chamber is surrounded by thefilter element.
 11. Apparatus according to claim 9, wherein the filterelement is tubular and the first chamber is defined within the filterelement.
 12. Apparatus according to claim 9, wherein the filter elementis tubular and the second chamber is defined within the filter element.13. Apparatus according to claim 4, wherein the inlet and the outlet ofthe collecting and shipping vessel pass through a common fitting near atop of the vessel.
 14. Apparatus according to claim 1, wherein theprecipitating agent is the trisodium salt of trimercapto-S-triazine. 15.Apparatus for removing silver from spent photoprocessing solution,comprising:means for providing a spent photoprocessing solutioncontaining silver to be removed; a mixing vessel; first means fordelivering the spent photoprocessing solution into the mixing vessel; asource of precipitating agent for silver; second means for deliveringsaid precipitating agent for silver into the mixing vessel; a source offlocculating agent for a precipitated formed by mixing the solution andthe precipitating agent; third means for delivering said flocculatingagent for the precipitate into the mixing vessel, whereby flocculatedparticles are formed by the precipitate and the flocculating agent; asettling vessel for receiving flocculated solids and any remainingliquid from the mixing vessel, the settling vessel having a slopedbottom wall and a bottom outlet for liquid and flocculated solids; aninlet passage within the settling vessel, the inlet passage extendingnear the bottom wall and having a length providing a residence timesufficient for forming clumps of flocculated particles of theprecipitate; an outlet for clarified liquid on an opposite side of thesettling vessel from the inlet passage and near an upper end of thesettling vessel; fourth means for delivering flocculated solids andliquid from the mixing vessel into the inlet passage; and beneath thesettling vessel, a collecting and shipping vessel having an inlet,releasably connected to the bottom outlet of the settling vessel, forreceiving the flocculated solids and any remaining liquid from thebottom outlet, for permitting the flocculated solids to settle to abottom of the collecting and shipping vessel and the remaining liquid tomove toward an outlet of the collecting and shipping vessel, whereby thesettled flocculated solids gradually will agglomerate and compact tofill substantially the collecting and shipping vessel while at least asubstantial part of the remaining liquid gradually will rise to anincreasingly higher level and eventually pass from the collecting andshipping vessel, thereby permitting a filled collecting and shippingvessel to be disconnected from the bottom outlet of the settling vesseland replaced with an empty collecting and shipping vessel.
 16. Apparatusaccording to claim 15, wherein the settling vessel has a cross sectionand the inlet passage is defined by a baffle wall extended across achord of the cross section of the settling vessel, the baffle wallhaving a lower edge near the sloped bottom wall.
 17. Apparatus accordingto claim 15, wherein the outlet of the collecting and shipping vessel isabove the bottom of the collecting and shipping vessel; and theflocculated solids and any remaining liquid flow into the collectingvessel near the bottom of the collecting vessel, whereby any remainingliquid flows upward through previously settled solids, thereby removingfines before the liquid reaches the outlet of the collecting andshipping vessel.
 18. Apparatus according to claim 15, wherein theflocculated solids and any remaining liquid flow into the collecting andshipping vessel near the bottom of the collecting and shipping vessel,whereby any remaining liquid must flow upward through previously settledsolids, thereby removing fines from the liquid; a tubular filter ispositioned within and extended upward from the bottom of the collectingand shipping vessel, the tubular filter having an interior plenum; andthe outlet of the collecting and shipping vessel is connected to theinterior plenum.
 19. Apparatus for removing silver from spentphotoprocessing solution, comprising:a mixing vessel; first means forproviding into the mixing vessel a spent photoprocessing solutioncontaining silver to be removed; a source of precipitating agent forsilver; second means for delivering the precipitating agent into themixing vessel; conduit means, defining a mixing path and having a crosssectional flow area, an inlet end and an outlet end, for receiving andpassing mixed solution from the mixing vessel; a source of flocculatingagent for a precipitate formed by mixing the solution and theprecipitating agent; third means for delivering the flocculating agentinto the conduit means; the outlet end of the conduit means beingdownstream of the third means for delivering by a distance providing aresidence time sufficient for forming clumps of flocculated particles ofthe precipitate, the residence time, the precipitating agent and theflocculating agent being such that clumps of flocculated particles growand agglomerate during movement of the solution and clumps through thedistance into enlarged clumps many of which individually extend acrossand substantially fill the cross sectional flow area, whereby individualenlarged, moving clumps substantially block passage of and strain fromthe solution remaining fine particles which then adhere to the enlarged.moving clumps as the enlarged clumps move to the outlet end; and asettling vessel for receiving clumps and solution from the outlet endand allowing the clumps to settle from the solution.
 20. Apparatusaccording to claim 19, wherein the settling vessel comprises:acollecting and shipping vessel having an inlet, releasably connected tothe outlet end of the conduit means, for receiving the flocculatedsolids and any remaining liquid, for permitting the flocculated solidsto settle to a bottom of the vessel and the remaining liquid to movetoward an outlet of the vessel, whereby the settled flocculated solidsgradually will fill substantially the vessel while at least asubstantial part of the remaining liquid will pass from the vessel,thereby permitting a filled collected vessel to be disconnected from theoutlet end of the conduit means.
 21. Apparatus according to claim 20,wherein the outlet of the collecting and shipping vessel is above thebottom of the vessel; and the flocculated solids and any remainingliquid flow into the vessel near the bottom of the vessel, whereby anyremaining liquid flows upward through previously settled solids, therebyremoving fines before the liquid reaches the outlet of the vessel. 22.Apparatus according to claim 20, wherein the flocculated solids and anyremaining liquid flow into the collecting and shipping vessel near thebottom of the vessel, whereby any remaining liquid must flow upwardthrough previously settled solids, thereby removing fines from theliquid; a filter element is positioned within the vessel, the filterelement dividing the interior of vessel into a first chamber forreceiving flocculated solids and liquid and a second chamber forreceiving liquid passed through the filter element; and the outlet ofthe vessel is connected to the second chamber.
 23. Apparatus forremoving silver from spent photoprocessing solution, comprising:a sourceof precipitating agent for silver; a source of flocculating agent for aprecipitate formed by mixing a spent photoprocessing solution and theprecipitating agent; means for combining a spent photoprocessingsolution containing silver to be removed, the precipitating agent, andthe flocculating agent; a conduit, connected to receive solution fromthe means for combining, the conduit having a length, a cross sectionalarea, an inlet end and an outlet end; means for delivering solution at aflow rate from the means for combining into the conduit; the length ofthe conduit and the flow rate through the conduit being chosen toprovide a residence time sufficient for forming large, ripened clumps offlocculated particles of the precipitate; the residence time, theprecipitating agent and the flocculating agent being such that theclumps of flocculated particles grow and agglomerate during movement ofthe solution and clumps through the distance into enlarged clumps manyof which individually extend across and substantially fill the crosssectional flow area, whereby individual enlarged, moving clumpssubstantially block passage of and strain from the solution remainingfine particles which then adhere to the enlarged, moving clumps as theenlarged clumps move to the outlet end; and a settling vessel forreceiving clumps and solution from the outlet end and allowing theclumps to settle from the solution.
 24. Apparatus according to claim 23,wherein the means for delivering solution provides flow rates into theconduit to provide a Reynolds number less than about 4,000 for flow inthe conduit.
 25. Apparatus according to claim 1, wherein the crosssectional flow area of the conduit means increases between the inlet endand the outlet end.
 26. Apparatus according to claim 25, wherein thecross sectional flow area increases stepwise.
 27. Apparatus according toclaim 25, wherein the cross sectional flow area increases gradually. 28.Apparatus according to claim 1, wherein the conduit means is definedbetween two sheets of material sandwiched together.
 29. Apparatusaccording to claim 28, wherein the sheets are made from thermoplasticmaterial.
 30. Apparatus according to claim 28, wherein one of the sheetsis transparent or translucent.
 31. Apparatus according to claim 28,wherein the cross sectional flow area of the conduit means increasesbetween the inlet end and the outlet end.
 32. Apparatus according toclaim 31, wherein the cross sectional flow area increases stepwise. 33.Apparatus according to claim 31, wherein the cross sectional flow areaincreases gradually.